Heh, I made the reverse choice that the OP did, switching from Android (Galaxy S3) to WP (Nokia Lumia 925). Happy so far.
Either way, best rom I can suggest for Android is CyanogenMod. That's just me though. You should also keep an eye on Ubuntu Touch.
That's exactly what my gf did and what I'm planning to do too (she switched her S3 to Lumia 925, I'll do the same with my Galaxy Note 1). No matter which OS you use, you'll end up wanting something new or different. Well, at least I do, but what do I know - I'm a nerd.
That being said, you really can't go much wrong with Android. It has all the apps you need, it's a pretty mature OS etc. Like someone said before, XDA is your friend. Another good place for hints and tips is Modaco. There are loads and loads of different roms (at XDA) to choose from (I liked Paranoidandroid very much, but always ended up coming back to the stock rom, but it's fun as long as you don't brick your phone and turn it into an expensive paperweight).
Must apps (a few of them) that I have on my Note are: Whatsapp, Spotify, Netflix, OneNote, SkyDrive, CallerInfo, Sports Tracker, Flipboard & Zite, Swype&Swiftkey (can't make up my mind which one is better, both are better than Note's stock kb which is r-e-a-l-l-y laggy...but I'm sure that you won't have that problem as your phone has better hw...Even though it's a bit funny how a 1,4 GHz dual-core cpu can't cope with the stock keyboard properly....I guess It's a h-e-a-v-y OS = it's like a train equipped with Porsche 911's engine...but now they've moved on to using jet engines, so no more troubles there).
Anyway, good luck with your new phone. I'm sure you'll like it!