The S2 beats the 4s in my book. Usability wise if you’re used to an iPhone then it will seem odd, pretty much like if you’re used to Windows a Mac will seem odd.
The S2 and HTC One X are probably the best two phones available. The One X is newer and has a bigger screen, faster etc. but ropey battery life. I’d go for an Orange Panther plan, simply because swappables are great. Best deal I could find as a new customer on Orange was an S2 on Panther 26 (£26). As a renewing customer, I managed to get them to knock a tenner off one of the dearer Panthers for myself.
Apps do vary between the two – iPhone tends to be the platform that makes the developer more money, apps tend to be paid for and plentiful. Android also has loads of Apps, lots of them can be had in free versions with ads, but I’d say the app market tends to lag slightly behind as a rule.