There are way too many reasons to NOT buy the PS3 version. I bought both the 360 and PS3 versions, and the only thing that the 360 version has against it, is that the guitar is wired.
Some would argue, however, that wired guitars are more accurate. And besides the wireless 360 guitar comes out next month (April 15th? correct me if I'm wrong)
I returned my PS3 version for many reasons:
-In Canada, they make us pay $2.99 PER SONG for DLC. Despite the fact that our currency is pretty much on parity with the US dolla bill. Not only that, but I hate giving my credit card out (I'm sure the PSN is safe and whatnot, but I'm holding out for when Sony starts using prepaid cards in North America)
Yeah it's a hastle to keep my MS points topped up... but I go to Best Buy on a regular basis, so it's not THAT much of a hastle.
-I have a 40GB PS3, and therefore I only have 2 USB ports. Rock Band requires 4 USB ports (for the full gaming experience..whenever a 2nd guitar is available) or a hub (an EXTRA $30 for a decent powered hub).
The 360 version comes with the hub, and despite needing it on the 40GB PS3, the PS3 version does NOT come with a powered hub.
-Online play: There are more people with the 360 version, and so there are more people to play against.
Also, the 360 matchmaking, voice and other LIVE things are simply better then the PSN Rock Band equivalents. That's not a bash against PSN. Harmonix just didn't take the time to implement all the same things for the PS3 version.
There are a couple of more things, but I don't have time to go into them all. I'd suggest going to the rock band forums (
www.rockband.com/forums) and see what kinds of +s and -s there are.
The only real downside to the 360 version is that if your 360 is vulnerable to RROD (mine is in a year-round constant 13 degrees celcius basement, so no issues with my launch 360), then the hours and hours of play time may take it's toll.