there's no best phone in the world. And it's difficult to compare this 2 due to different OS.

Best way to find out is to give ur time and play with both of the phones..
For apps, go for iPhone (if u really wanna get iPhone, wait for the next gen).
For customisations, go for SGSII.

Having used both.. there's no doubt iPhone is fluid and with tons of apps.
But I get bored after some time quickly with the apps and folders on the page (which I couldn't tahan)
With Android, I see different homescreens with widgets, picture frames, scrollable wallpapers, etc. And if I need to change sth, be it themes or system files those tweaks, I just google and the possibilities are there with Android.

iPhone - learn to jailbreak, use iTunes
SGSII - learn to customise for ur own liking

P/S: Don't even compare the screen. SGSII wins hands down.. (unless u gonna pinch-to-zoom and look at every alphabets in a book)