They are all good. If the amount of music and HD matters more then go for 360 since it has the largest total amount of songs available for download over the PS3 (slightly less) and Wii (slightly less again). Bare in mind a lot of the songs missing from the latter are mostly the free Neversoft songs which might not matter to you so much if you don't care for them. Bare in mind though that to access all the online features you'd probably need to pay for a Gold subscription on live. I'm personally not sure how limited it is though if you don't have it.
PS3 has slightly less songs than the 360 and free online, HOWEVER their online was recently hacked and could therefore be untrustworthy. It's really up to you.
Wii has free online, a fantastic interface and plays largely the same, just not in HD. Online works and resembles its HD cousins a lot, but won't obviously look as good and lacks the ability to voice chat.
They are all largely comparable so it really depends one which you own already, if any, and what factors matter to you more.