i say ps3
The Wii is cheaper but if u got the money for the ps3 then get it

ps3 graphics in rockband is terrible but ive played both versions and there the same if you play with the guitar but id go with ps3 because of all the games and graphics

Does the Wii even have the DLC for rockband??? If not then its a given..

Wii:
+ Cheaper Console
- If you are interested in GH:WT, the RB instruments are NOT compatible on Wii

PS3:
- More expensive Console
+ RB Instruments work with GH:WT
+ You can import all RB1 songs into RB2 for cheap if you have the disk

RB2 for the Wii does have DLC (RB1 did not).

They are slowly releaseing all the older DLC that the PS3 version has, but it might take a while for it to catch up.

Here is the list.

For just Rockband PS3 could be a bit better depending on your uses.  If your going to download new songs that is a PS3 advantage, Wii you have to buy additional disk based Track packs.  For older disks it might actually be cheaper here for Wii, as you can probably get them used, but the diversity here is lower on Wii and slower to come out on Wii.  If you have a great stero system or home theatre audio setup the PS3 will also blow the Wii away.

 The bigger issue is your looking for games like Mario Party in addition to Rock Band then you want a Wii.  I do not even know of any games like that which are on the PS3.  If you can find an old PS2 compatible one then you could get a lot of those types of games on it but on its own that is not an area the PS3 has much in.

DLC and Audio Quality goes to PS3.

But I would go for the one with the most games that I want.

Guitar Hero! :)

I don't think that graphics really maters in rockband but I urge to consider other games for each system that you want to play and then consider the cost and benefit factors.

If you only want a PS3 or Wii for this game, then get the cheaper one, obviously.
If you plan on playing more games eventually, get the one that has games that interest you more. From what I've read, it's Wii.