OK so the S3 vs S4 comparison here is a pretty comprehensive breakdown of the differences:
https://www.youtube.com/watch?v=SLwo4gPxUek
Essentially I see the differences being:
- slightly bigger screen from 4.8" to 5"
- slightly better resolution
- slightly better game performance
- slightly thinner/narrower design
- slightly better battery life
- introduction of sensor
- slightly better software
- more or less the same camera
To me it comes down to what type of user are you.
Are you likely going to be doing a lot of gaming? I currently play a lot of Real Racing 3, and yes to be honest there are times when it lags, but it's probably only about 5% of the total playing time. But I only play casually. If you'll be doing more on it then may be go with S4.
If you're just an everyday user who uses it for Facebook, Line, messaging, phone calls, non-graphic intensive apps, then I'd definitely just stick with the S3 - which is quite a bit cheaper. (I'm currently in this camp ATM). Moreover, because there isn't much byway of the camera quality, S3 is more than sufficient for what I use it for. I don't care about Samsung's gimmicky software like S-Voice, S-Beams, etc, at all.. it looks cool on paper, but in real life everyday use, I just never have time for them.
If you're looking for an above average smart phone, which is plenty quick for your everyday use, can take decent quality photos and something you can plug straight into your HD tv (iPhone can't do this) - then for the price, I think the S3 is a great option.
To me I don't see a whole lot of difference between the S3 and the S4. But if you're like me, and just want that additional use out of your phone (can double as a note pad) - then I'd go with Note II with the 5.5" screen with the build in stylus. I just think that it gives you a totally new use of the phone.