Some of the newer Android phones don't have SD slots anymore - which for me is a bit limiting.
I love that for £30 I could get a 64GB microSD for my S2.
If 16GB storage is enough, the Nexus 4 looks very good - and sim-free for £280 is a bargain.
As I've said before, I really like my S2, it was simple to root and install the latest version of Android - for me the software it ships with isn't very nice, and I bought mine specifically with a view to upgrading it myself.
Which some may see as sad/geeky, some may see it as a nice way to get the phone I really wanted (storage, software, price etc).
Personally, if you liked the iPhone and have other iDevices, I'd just get another one...