Why'd you choose the PS3 version out of interest? The 360 version has some free DLC and was patched to allow more of the song content to be played off the bat (one of the major criticisms of the game IIRC). What advantages swayed you the other way?
The DLC doesn't matter too much for me (most of that is available on PSN too) and I can wait for the PS3 patches
(DLC works, so that will work too).
Besides, there are cheats available already for unlocking all the songs.
The best thing about the 360 is apparently that you can pick someone from your friend's list to play against, whereas the PS3 version only allows you to play with/against others randomly. I'm hoping they'll patch that in the future, but I don't know if they will. This almost swayed me towards the 360 version, but I wanted to try the PS3 version anyway because:
1. my PS3 is in the living room, and this is a game I'd like to play in the living room, and have fun with with other people in the living room. Both picture and sound are much better in the living room, for obvious reasons. Also more comfortable.
2. the 360 is far too friggin loud (partly why it's not in the living room) and if there's anything that I hate that particularly with, it's during a game that I have to actively use my ears. In other games, I can use sound to block it off a bit, but not a music game.
3. I guess as with so many other people, I have a 'basic' system that I'm most comfortable with, have most of my real-life friends on*, like playing the most with - and for me that's the PS3. I like playing a bit of Guitar Hero, then switch to Warhawk or a PS Eye game / Eye of Judgment, play the latest demoes, browse the web a little, watch a DVD/BluRay, on the same machine.
Another possible regret may be that a wired guitar may just work best with this game, but I'm hoping this will be fine as wireless is nice. It's a bit ironic though that I partly got the game because I noticed I enjoyed the demo on the 360 even without the guitar (I've always liked Bemani games) and also that the wireless guitar uses a system that's exactly like the wireless 360 controllers. Understandable, but a bit silly nonetheless.
I'm still looking for a great game to buy for the 360 that I can play while my wife is rewatching Buffy or Angel for the 100th time (currently she's watching some bonus material), but I'm sure I can find an exclusive for that (eventually). Maybe Mass Effect, if you can skip the shooting parts (I thought I remembered from long ago you might be able to let your squad take care of most of that?), or something similar.
Anyway, I'll be sure to let everyone know here if I regret getting the PS3 version and why (not).
* I don't quite understand it, and it's probably because I'm a little bit older (33), but I don't know anyone else with a 360. We are five at work now with a PS3, and I know 1 more guy with a PS3 outside work. Then there is one PC gamer, and that's it.