I went from an SGS2 to a 4S because my wife bought me a 64GB 4S for Christmas

Before I got the 4S, I was perfectly happy with the SGS2 but I have to admit the iPhone
is a better phone, or rather that iOS is, overall, better than Android.
Things I prefer on the iPhone / iOS:
1) Lock Screen / Notifications
2) iTunes integration (with iTunes match)
3) The Browser
4) Email App - I used the GMail app on my SGS2 for my personal e-mail which was pretty good, but the mail app on iOS is just nicer to use (if not as powerful)
5) General look and feel of iOS and 3rd party apps: Android is miles better than it was, but it still lacks the consistency of iOS
6) Build quality. There's nothing wrong with the SGS2, but the iPhone just feels more "premium"
Things I prefer on the SGS2 / Android
1) A proper file system - not a massive issue, but it was nice to be able to sync files from Dropbox onto my phone in a central location
2) App Store. Not the content, but the web interface with remote installation to my device was much slicker than the iTunes interface for iOS
3) Battery Life. Yep, the battery on my SGS2 lasted longer than my iPhone 4S
4) Screen - I love the AMOLED display of the SGS2 and although a lower resolution than the iPhone, it really is stunning
5) Better separation of email accounts (i.e. allowing 3rd party mail apps so I could dedicate a separate app to my work e-mail)
Overall, I don't think there's a huge difference between the SGS2 and the 4S and a lot boils down to personal preferences between iOS and Android. I probably wouldn't have bought a 4S myself and stuck with the SGS2 for another year or so, but I do have to say I
do prefer the 4S to the SGS2. I couldn't give a specific reason for this beyond it just being a nicer device to use
for me