I've had the Samsung Galaxy S2 for a few weeks now and absolutely love it to bits.
I also had a look at the iPhone 4 and to be honest I liked the larger screen on the SGSII, it's looks and it's lighter too. I find the app's run brilliantly and it's super fast/stable.
My friend has the iPhone 4 that I played with and although he likes the SGSII hardware he purely wants the Apple OS. Which is fair enough. Me? I'll stick with the Android OS but on a premium handset like the SGSII Android feels so slick and reliable that it really about the OS you want to use.
I think it's purely down to which OS you prefer although if you do fancy making comparisons you should, at least, compare the iPhone to a premium Android handset.