Same as the ching above, I've tried both, for me, I prefer GS2, good for people who love to be free (personalize) and love new stuffs(won't be bored if you can change ui
or things on phone)
Also GS2 has all the features on iphone eg. Find and erase phone by a computer, HD video.....but samsung does even better.
For speed, Samsung is better, the cpu is faster and it is really faster
Bonus: 1.samsung kies air (wireless sync) works on mac and don't need driver, easy to sync 2. AMOLED of GS2 is AWESOMEEEE
Android market is not as good as appstore although samsung has samsung apps, but mostly only work on GS but not GS2.
Hot, uses quite a lot of bettery:) need to prepare external battery or a spare one.
User friendly: BOTH
Music Player:ip4(coz GS2 Isnt strong enough tpower my 31ohms Westone 4)
Video playing and filming: gs2
Web browsing (mobile site): ip4
Web browsing (normal site include flash and java): gs2
Word input: gs2