go find an iphone 4 and play with it
next, find a SGS2 and play with it (or vice versa)

go for the one u like.

For a person who used both,
SGS2 is a great phone, but i heard too much about heating problem... other than that yup, its superb
Iphone4 is easy to use, but u need to grab onto Itunes, and learn how to jailbreak for more freedom, but being an android user, i still envy the number of apps and games on the iOS

Dont always compare specs, OS, functions, and so on, go with ur heart...
what do u really need? camera? apps? games?
do u like to customise ur phone?

and yes, it is not fair to compare SGS2 to IP4, like everybody says (except for Similan) SGS2 is on a different level, minor lags are there becoz of the multitasking of android, iOS uses a different method
only fair way to compare, IP5 vs SGS2/Note

@Similian, who says SGS2 is done to counter IP4? Then how bout we say ipad2 is to counter SGS?

TS, make ur own choice, try it on ur own, no point comparing this way.