From what I hear GS3 is out in March and has a 2.2ghz dual core processor
- might just be a rumor though.
As for screen size, iphone owners tend to say the s2 screen looks massive, where as their screens look tiny to me. The s2 is super thin, so you don't really notice the size in your pocket. I'm sure neither is gonna get on your nerves once you get used to it, but having a slightly bigger screen is great for browsing/games/films imo.
As for being flimsy, I've had it for over 6 months of heavy usage, without using a case, and it still looks and feels like new. I've never had that with previous phones or my ipods.
Another thing is the price, you can get an S2 for much cheaper than a 4s.