I actually just switched from 3+ years on Android and a Galaxy s2 (most recent phone) to ios and the iphone. And I actually am using the original iphone 4, no s. As others have said, it really comes down to preference. Since you have experience with both platforms, it should be relatively easy for you to decide which ecosystem you prefer. For me, I've been a mac user at home for a long time, but I've also been a t-mobile customer for a long time, so I've been using Android. But the itch to go ios finally broke me and I am jumping ship at t-mobile and using the iphone 4 on straight talk (and saving $50/month while I'm at it).
Anyway - for me, the s2 screen size was almost too big. I wouldn't mind something a little bigger than the iphone, but I'm in the camp of believing that a phone shouldn't be TOO big; it is, after all, still a phone and should be a compact pocket-size device. So many times, I would have to awkwardly shift the s2 in my hand to try to reach a key on the other side of the phone. One-handed use was a challenge at times (ymmv depending on use and hand size). Also another things that drove me crazy with the s2: the soft hardware buttons below the screen. I would CONSTANTLY hit the back or home buttons while typing without meaning to. Started to drive me crazy.
Hardware aside, the major difference is ios vs android. For me, I was tired of Android, which worked well but always seemed a little glitchy - so far, I'm very happy with ios and how well it integrates with my mac at home. Also, LOVE having garageband on the go!!!!! (one of my personal BIG reasons for switching).