I've had the opportunity to play around with both for a significant chunk of time and this is what I see as the main differences...
- Galaxy S2 is more of a "pure Android device" - less cheap thrills like spinning menus and pretty weather widgets you'll probably get sick of in 6 months
- Galaxy S2 is faster (though this doesn't mean it'll always run its UI faster) and has more RAM and storage
- Galaxy S2 has a brighter, crisper screen that can be viewed from more viewing angles. This is just incredibly apparent all the time. The Galaxy S2 screen is gorgeous.
- Galaxy S2 has no fancy lockscreen allowing you to easily access the phone's camera, messages, dialer, etc, which is a drawback, butt...
- Galaxy S2 has a better battery life. Much better, in fact.
So there you have it. Those are my impressions so far..