Here's the breakdown
Processor:
iphone 4 : 1.0 Ghz
Galaxy S : 1.2 Ghz dual core
Camera:
ip4: front VGA

back: 5 mp
gs2: front 2 Mp back 8 MP
s2 has a larger size screen, on a side by side comparison the s2 looks more bright, i almost thought it was a sticker on the screen!
s2 is slimmer
s2 has expandable memory, where ip4 you are stuck!
S2 gives more bang for the buck imho
If you want the iphone specific apps/games, then theres no other option.
I'd say galaxt s2 !