A few more pros for each:
Xbox - there is a much larger online user base for RB, so finding people to play with should never be an issue (this only really matters if you play with randoms online, if you don't play online, or only play with friends then its moot). ...on the other hand (and this goes hand in hand with the larger base and judging just by what is often complained about here on the forums), the Xbox user base seems to have (warning, huge generalization headed your way) younger and possibly less mature players.
PS3 - ability to easily upgrade to a larger off the shelf hard drive. Which could come into play as you increase your DLC count (although if you only plan on buying RB/RBN DLC, I don't know how large a hard rife you'd need to store the 29xx song limit anyway).
Lawdog - "That may be the dirtiest thing anyone has ever said on here."
I'd like to add something that I can't confirm since I'm not a xbox player, but If you're planning on playing pro, keep in mind that if your band mates doesn't have the upgrades, you won't be able to play the songs that you have the upgrades and they don't, no matter which instrument, pro or not, you are using. (Someone on xbox should be able to confirm this though, I'm not 100% sure but I've seen complaints about this.)
This is fine on PS3 though, I can confirm that I can play those songs with people that don't have the upgrades.
Expert Pro Keyboardist
Expert Pro Bassist
another 1 for xbox, no dongles for your instruments which can break/get lost
my love is like a candle, if you forget me, i will burn your damn house down.
All PS3 models except the original 60GB version are terrible.
Wishlist (possible artists):
 The Killers
 The Cure
 Sum 41
 Linkin Park
 Green Day
 The Offspring
Reliability is the big issue with me. I've never had an Xbox NOT RRoD on me and I've gone through 5 of them before I decided to do away with them forever (both the original Xbox and the 360). I don't like buying something that's a ticking time bomb and that's exactly the way I feel about Xbox. Now, flipping the coin, I've only had 1 Playstation console break on me (my original PS2 stopped reading discs), and that's it. I have a slim PS2 that's going on 7 years old, a fat PS3 going on 5 years, and a PS3 slim going on 2, all running as strong as they were the day I bought them. I like worry-free hardware that's reliable, but again, that's just me.
Guitar Covers Channel: www.youtube.com/user/MysticGuitar77