I had an iPhone 3gs, iPhone 4 and an iPhone 4s since launch day. I moved over to this phone a month ago, and I officially gave up my 4s when I activated it on my wife's line last weekend.
Battery - if you feel that the 4s battery is bad then you will be disappointed. Like another poster stated, I end up with about 25% at bed time, where my 4s was around 50%. The good news is, you can buy extended batteries for this phone. Just watch out for the cheap knock off batteries. If it sounds too good to be true, IT IS!
You also must remember that both are true smart phones, unlike the BB. The high powered screens, 4g speed, multi-tasking, etc all lead to more demand and less battery life.
Apps - You can find almost all of the same apps on Android as on iOS. Many more are free, but some are of poor quality. Make sure you read reviews and always try free versions first if available.
Pictures - I had Photo Stream and don't miss it. I use Picassa / Google+. All phone pix sync with this service. Make sure you set it up and sync when on wifi. Mine used 2gb of data on day one from syncing pictures.
Music - I use iTunes and have no intentions of switching. I use an app called iSyncr with the wifi add on. This is not a free app, but well worth it. You can sync your entire music library and/or individual playlists wirelessly. I have mine set up to go at 2AM. It also uploads my photos to a folder that you can import into iPhoto. (Yes, I use a Mac) It is 2 way sync, so music purchased on your phone can be synced up to a folder on your Mac too. You simply have to add the songs to your iTunes library.
There are other apps as well, Double Twist, SyncMate, etc. that I have heard good things about. I tried SyncMate and it allows some additional features like iPhoto, calendar, music, video syncing, but I found it problematic.
After a month with the GS3 I am very happy. I actually cancelled an order for an extended battery just yesterday. I decided that I am getting by on this battery and it's too easy to top it off in the car or at my desk if necessary.
I hope this helps you in your decision or to answer your questions as you come over to Android.