[LG Optimus 2x] TheStonedDog:
There might some issues with comparing a "S2" just in that there are half a dozen variations that carry the S2 name with international, LTE, HD, etc. versions. I am going to assume that the OP is referring to the original i9100 4.3" version and the P990 2X.

I'll come out of the closet and admit it, a couple of weeks ago I bought an i9100 S2. I have both the 2X and S2 sitting in front of me right now. I was even carrying both phones around with me for a short time which made for some strange looks when I would whip out two phones. I will list my thoughts and experiences with both. Some of these observations will be subjective "feel" type so take them for them for what they are worth.

The S2 is bigger particularly in width but it is also thinner. It took awhile for me to get used to it as I like having some thickness for my fingers to grab but with the wider display then thinness made it easier for my fingers to reach around. I also use my phone one handed alot and the S2 thinness makes it easy for my thumb to reach the edges of the screen. I tried a S2 HD which has a 4.6" screen and is thicker for the larger battery and its just on the edge of not being usable with one hand. I also just tried a S3 and its pretty much impossible to use one handed. You definitely need to move the phone around in your hand to gain access to all the screen area. 4-4.3" seems to be a sweet spot for me. The 2X I have no trouble moving around the screen. The beveled glass sides feel nice.

The 2X seems to have thicker glass. I think this is backed up by me never seeing a broken 2X display online for sale while I have seen many S2s with broken displays for sale. I'm not saying the 2X cannot be broken but it seems to happen less often. I like the 2X has an integrated camera lens protector in the cover even though it does collect dust and you have to clean it. The S2 camera lens is fixed so if you damage the lens say by scratching in your pocket then you have to replace the whole camera. The camera bump on the 2X is much more pronounced then the S2 which I didn't like. The cover on the 2X is harder to get off because it is thicker and the matte finish is not bad but collects marks and can be slippery at times. The gray coating around the 2X bezel also tends to chip off make the phone look abused. This is the only phone I have had this problem with as I do take good care of my phones. The S2 back cover is thinner so it comes off easier making battery changes a snap. When installed the S2 cover is recessed with the case so it doesn't not feel thin at all. The S2 textured finish makes the hold nice. I haven't not witnessed any chips on the S2 finish. Build quality I with both is the same but if I were to pick I would say the S2 is better.

The 2X is also heavier. The S2 is very light even though it has a bigger battery. I think the 2X weight is a combination of the thicker glass and thicker plastic parts. I also think there are some cast aluminum parts in the 2X. I would sometimes get hand fatigue holding the 2X for long periods. With the S2 I don't suffer from this. Also when I carried the 2X in my jacket it would drag my jacket down on the one side. I had to move my wallet, keys, and other bits to the other side to balance the 2X. Again not an issue with the S2. I think some people when they pick something up think that lighter is cheaper or weaker. Not in the case of the S2.

When I moved to the 2X my previous phone had physical buttons. I was not sure about the all capacitive button situation. I ended up liking them as they did not collect dust or get miss triggered in my pocket. Finding them in dark conditions was not much of an issue as they lit up when I hit the physical power button. The S2 home button does get dust but its nice to be able to hit something on the front to turn the phone one instead of having to reach around the top and hit the power button. It also does not suffer from miss triggers from a light finger brushing by during normal use like with the 2X. The physical button does break up the other two capacitive buttons reducing accidental hits which with the 2X having the four buttons close together sometimes happened. I almost never use the search button on phones so on the 2X I had it set to open my music player and bluetooth controls when held. Nice but not missed either.

With the 2X I could get by a day with regular use maybe two with light use. On the S2 I am breaking in some new batteries going through full charge and discharge and I am having trouble draining them in a day even with heavy use. The 2X has a 1500mAh and the S2 has a 1650mAh so there is not much difference. I do have 1800mAh batteries for the 2X but I think they are overrated. I have 1900mAh batteries for the S2 but I've see the label taken off this model and the factory printed info has a 1650 on it so I would say they are still 1650mAh and are overrated. So even though I have supposedly larger batteries for both the S2 still wins.

For software support the S2 wins hands down which is one of the reasons I went with that model. With the expectation of the Nexus series the S2 is one of the most supported devices out there. It already has original official factory and CM nightly ICS builds. JB is coming soon as well. The 2X on the other hand...well you know.

On the topic of stock software I do not have a common platform to perform a direct comparison but this is what I felt. I only ran the stock Froyo on the 2X when I got it for a few days and went straight to CM7 so I don't know how the stock GB was but the stock Froyo was just trash. My S2 came with ICS so I can't compare GB builds but the stock ICS was not too bad. I did not like the Touchwiz skin but there were some nice features included and the response was decent so if I dropped another launcher on it I think I could live with it. LG not so much. So I think Samsung has better stock software then LG.

I do like to spend time "dialing in" my devices so this is my preferred setup on my 2X. I currently run temasek's CM7 (I think it is currently build 132) with vadonka's latest ETaNa v2.6.32.59 kernel. I overclock to 1.4G as my phone is rock stable at that with 32M ram hack. I run GO Launcher EX but near the end I was switching between that and Holo launcher. With this setup I was very happy with the performance. This was backed up by the benchmarks numbers I was getting.

When I got my S2 I was bouncing between the two phones and honestly at one point early on I started to get some remorse as my custom 2X felt better then the stock S2. I began to question my purchase. Now that I spent a couple of weeks dialing in my S2 (CM9 nightly, custom kernel, Apex launcher, and a few other tweaks) it is definitely much better then the 2X. I do overclock to 1.5G but my readings show that the phone rarely ramps up to that speed. I can run at the stock 1.2G and still be happy. It feels like while both phones are of the same generation the 2X is suffering from being an early adopter without the benefits of hardware refinements while the S2 is an experienced hardware version. Now one could say this is GB vs. ICS but we can only compare on what is available at this moment. So IMHO a custom 2X will beat a stock S2 in normal usage but a custom S2 will beat a 2X no matter what. The benchmarks I and others have done clearly show the S2 beats out even my custom 2X. No question. S2 wins.

The S2 screen has great contrast and is sharp. It does suffer from slight burn in. My wallpaper is black with white graphics so when I switch into an app I can see a slight faint residual image. It does go away after a minute or so. The 2X LCD does not suffer from this problem. The 2X does get alittle washy with the backlight screen especially on bright coloured images. The S2 is very readable in sunlight where the 2X sometimes needed to be read in the shade. S2 for me but some may still like the LCD.

The 2X has a dedicated HDMI port which is nice for playing games. The S2 needs an USB adapter and only works on the stock ROM as Samsung has never released the drivers for the HDMI. Boo. 2X wins.

The camera on both are good. I'm not a phone camera nut but both seem pretty acceptable to me. The S2 can record HD at 1080p though without trouble where the 2X would struggle but I don't do much video. I call it a wash but S2 if you really need it.

The SD card on the 2X is easily accessible. On the S2 the SD access is blocked by the battery so you have to turn the phone off when changing SD cards. No biggie but something to keep in mind if you change SD cards. I noticed the Samsung phones afterwards have easily accessible SD cards so it might have just been with this model. 2X wins.

There are other problems with my 2X that maybe due to GB like the bluetooth is flaky, can't sync the clock to the provider, the light sensor adjustment was poor, the audio jack output was weak (I did use voodoo to compensate), the occasional BSOD when I ran K-9 in the background, etc. The S2 has none of these issues. In fact I don't think I have run into one problem with the S2. I don't know if it is CM9 or the S2's hardware but the fact is it works while the 2X has issues. The S2 wins.

When you look at the specs on paper the phones are not that far apart even against the newest crop of phones but when you get into the phones' usage the S2 shines brighter on most accounts. With both phones at my disposal I am sticking with the S2. I cannot wait for CM10/JB to see what this phone can do with that and it will see it since the S2 is much better supported then the 2X. With the price difference though the 2X is probably the best phone in it's current cost grouping especially if you can put the time in to tweaking it and who knows it may get another boost with a CM9/CM10 build.

TL;DR version is the S2 is a better phone but the 2X is not bad either for the lower price.