[AT&T HTC One (M7)] jtc276:
I wrote this for another thread but it's basically my main impression on the whole "S4 vs One" topic.

Ok. Take in mind that this is from someone who was dead set on getting the S4 only a week ago. I also want to add that my non-tech-savvy iPhone-using mother was with me at the AT&T store while I was comparing phones and also agreed that the One is better.

1. The screen is smaller on the One but it has better color reproduction and, when playing the trailer for 'Star Trek Into Darkness' on both devices, produced clearer results.

2. The One's software is much more optimized and runs smoother than that on the S4. In my half an hour of constant use with both devices, the One maybe had three instances of minor lag while the S4 produced lag (sometimes major lag) every few seconds. The S4's lag was most prominent when multitasking, hitting the back button, or going home. Switching between home screens produced some stuttering but was not terrible.

3. Speaking of software, the "features" on the S4 aren't that great. Probably the most difficult thing about using the S4 was trying out the Smart features. Many of them were pretty useless such as scrolling through web pages with hand gestures. However, it did actually work and work pretty well. I also really liked having the ability to answer the phone by waving in front of the phone and seeing some general information displayed on the phone after waving my hand in front of it while it was sleeping. But that's about as useful as it gets. I could never get Eye Tracking to work. Some kid beside me finally did after nearly a minute and a half of working with it which completely defeats the purpose of using it in the first place. None of the other features really worked well either making me wonder why Samsung decided to add them to the device and bog it down in the first place.

4. Build materials are pretty bland on the S4. I'm not going to criticize them for using plastic because plastic is durable enough, but I am going to criticize them for not really differentiating the phone from the S3 in terms of look. I was, however, impressed with how they implemented the 5" screen into such a compact and usable package, but that's about it. The S4 is no looker. My mom was also not a fan of the design, thinking at first that the hyper glazed finish was some sort of case and not the actual design.

5. Sound quality through the headphones was good on both devices but better on the One due to its added amplifier. The S4 had definitely improved over the S3 and maintained a pretty loud volume. The One was slightly louder and produced more dynamic results with great highs and lows. The average person would be happy with the sound quality from either, though.

6. The general Android apps such as Texting and Email are hardly changed on the S4. For some users, this will be fine. But, personally, I hate the stock Android Messaging app. I have a Nexus 4 and have used GoSMS since the day I purchased it. I don't understand why anyone would want to view a multiple picture MMS like a slideshow instead of each photo separately. Thankfully, the One does exactly that and has a rather perfect Messaging app. Of course, you can always download GoSMS or Handcent or something from the Play store, but the fact that this won't be necessary on the One is nice.

7. This is picky, but I really liked the One's keyboard. Like, a lot. I've been a stock Android keyboard user since ICS and I didn't think that choice would change with the purchase of any new phone. But the keyboard on the One is nice. Really nice. I almost never made mistakes on the keyboard when using it. On the S4, I couldn't stop making mistakes. If you're just going to use SwiftKey or the stock Android keyboard, this probably isn't a huge deal to you. But I found it shocking that I loved the One's keyboard.

So, yeah. This is a huge post but those are my honest opinions. I was as subjective as I possibly could be and my honest opinion is that the One is the better phone.