Maybe I can help since I've had the S2 since May or so and I've ordered a 4S...
Hardware wise, the iPhone just feels like it is a much nicer phone. More premium. The S2, and really every Samsung smartphone, and a huge chunk of Android devices are made out of plastic, so while it doesn't feel flimsy, it doesn't feel as solid as the iPhone. I also prefer the weight of the iPhone, the S2 is light, but almost feels too light.
Also, and this is more of a problem with Android, but even with the very powerful processor, the S2 feels fast for the first little while then starts bogging down and now it doesn't feel as fluid as it was when I first got it. I might have to do a restore to get it back to where it was.
Screen - The SAMOLED Plus screen is definitely the winner as far as contrast goes, but along with that it does come with over saturated colors. Seems very forced and trying to make the picture nicer, but instead it exaggerates the colors. The IPS display gives more natural colors and offers more PPI so text is sharper, which is noticeable, but really depends on if it matters to you or not.
Web Browsing - The S2 does render faster than the 4, but obviously with the A5, the 4S should be just as quick or even quicker. Android still has a long way to go compared to iOS in the scrolling department. It still feels choppy.
For me, the things I mentioned along with missing the iPod app and Apple integration are the reasons I am going back to iPhone. I really did want a larger screen, but part of me is still a bit happy with the 3.5" because I have smaller hands and reaching across the 4.3" screen of the S2 with one hand gets a bit uncomfortable. I'm stoked to get my 4S next week!
Hope that helps.