[AT&T Samsung Galaxy S II SGH-I777] Product F(RED):
Having owned a Galaxy S II (Sprint) and now a 4S (Sprint, Unlocked), I'll outline the pros of each (and Android itself vs iOS):

Galaxy S II

- Fastest, smoothest Android phone you can get right now, even compared to the Galaxy Nexus. "The iPhone of Android phones"

- Tons of ROMs, mods, hacks, if you're into that kind of stuff

- Faster camera lighting adjustment than the 4S's camera (smoother panning)

- Comes unlocked or can be unlocked very easily (GSM versions)

- 4.3/4.52 inch screen vs 3.5 inch screen

- SAMOLED Plus vs Retina (aka qHD on a 3.5 inch screen)

- Expandable storage; 16/32GB/64GB (<--rare) + MicroSD card slot

- Android out of the box = Jailbroken iOS

- Wifi works normally, in iOS it goes to sleep after the screen goes off unless you use Insomnia to keep the CPU running at max speed, thereby eating your battery.

- Overall a much more versatile OS if you love to tinker

iPhone 4S

- Sprint/Verizon version are unlockable for international use (real unlock)

- "Just works" philosophy, 'nuff said

- iCloud is great. You don't need iTunes anymore unless you wanna transfer music/photos/videos

- Syncs with Google just fine (Contacts, etc)

- Carriers cannot rape it with apps or make their own version of it that is different from the rest.

- Vibrate/Silent switch (not really a huge pro, but still helps)

- Smoother than Android, even the GSII

- Resell value is much higher - A 32GB Sprint Unlocked 4S goes for $700+ on eBay, and more if it's white.

- Glass + aluminum feels much better than plastic

- Much more accessories

- Much better audio quality (better DAC + speakers)

- Better notifications/notification bar (yes, they copied Android but it IS better)

- Cydia provides a

ton

of tweaks, not just apps that aren't allowed in the App Store

- It's always going to be $200 for the lowest memory model (of the current edition, in this case 4S)

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Now, as for why I switched. I don't think iOS is the better OS. In my mind Android will always be the best OS (at least for what I'm interested in and in terms of versatility). There is nothing you can't do with a powerful Android phone. However, after 2 years of messing around with Android (Evo -> Evo 3D -> Nexus S 4G -> Evo 3D -> Epic 4G Touch -> iPhone 4S), I got tired of having to flash ROM updates constantly to try and keep up with the latest versions. On top of that, I was getting horrible, explainable battery life from my GSII. I only ever got good battery on my Evo 4G, and that was back in the days of the Netarchy-Toast kernel and Myn's Warm TwoPointTwo. 15 1/2 hours on a stock battery, not too shabby. But I digress; first, and foremost, I got this 32GB 4S White Sprint Unlocked because it was a good trade for my E4GT. Secondly, I needed a phone for international travel. And thirdly, as I was talking about before, I wanted something that just worked. I get good battery life with this thing. It plays my music, fetches my email, has games, Facebook, web, etc. It's guaranteed updates for a long time, and that are on time, and overall just gets the job done. I was tired of fooling around with my phone every 3 seconds wherever I was. Am I gonna miss the 1-inch-bigger screen? Definitely. Expandable storage? Maybe; I had 16GB + 32GB card, but never filled up all the space. Freedom of Android? Eventually. But right now, for my needs, I'm fine.

Will I ever switch back to Android? Most likely yes. When a really compelling, international phone comes out (since I'm on Sprint and I travel here and there).