I'm listening to Spotify on my GS3 right now but sometime over the next two weeks I plan to return it and wait for the new iPhone. As far as power is concerned it's a truly fast phone. It would be faster if it were running stock Jelly Bean, but that's not going to happen with Samsung. Not while this crap called Touchwiz exists. Plus, I REALLY like Google Now and offline speech to text and I can almost guarantee that it won't make it to the GS3, not while S-Voice is here. S-Voice sucks. Siri wasn't much better, but with iOS 6 the integration was going to be more complete. Maps is a wash- both get you from one place to another. And in 3D now. On my 4s I was running a beta of iOS6 and I found Maps to be about the same across both platforms.
I have beef's with messaging. I enjoyed being on the group texts at work, the ones where we all sit in the cubes and be open, brutally honest and pretty nasty. And if you weren't in the "know" you never knew about it. You had to get invited. I liked iMessage for that. I've tried 3 different messaging apps and haven't found one that works as well or as smoothly. I'm running Apex as my launcher and it stutters occasionally. Probably because it's running over Touchwiz. The size of the phone is great, has great battery life too.
Partly, I blame my friends. They all have iPhones and I feel more connected thanks to iMessage and to be honest, I like the way Notes now syncs to Mountain Lion. I take a prolific amount of notes in my weekly meetings and I don't use Evernote, so the sync function is pretty big. The Galaxy S3 is a GREAT phone. I love it, but the Apple ecosystem has me pretty well in their corner. I also prefer iCloud and the backing up and syncing.
The best thing about all this is that I know where I stand. I tried the GS3 and have no negative things to say about it. It just didn't fit in with me as well as the iPhone and iOS functionality. So for the OP, it's just about deciding which works best for you. It's very easy to be swayed by a large, excellent display, tons of horsepower, superb battery life and a different experience.
As a side note- the 4S feels positively tiny now and not in a good way. Looking forward to a larger form factor. Too bad I gave the 4S to my wife. Have to use a very crappy go phone in the interim weeks leading up to the new iPhone release.