If you're looking for the best variety of downloadable content, the 360 would be the best route pretty much due to the Rock Band Network. There's over 1000 songs available on Rock Band Network and only a fraction of those are on PS3 and Wii. PS3 still gets a few songs each week, but the Wii isn't getting anymore RBN tracks at all. The PS3 has just about 200 total RBN songs right now and usually gets 5 new RBN tracks per week.
Visuals will obviously be better on PS3/360 over the Wii no matter if it's Rock Band or Guitar Hero.
Overall, the Wii may be the worst choice overall, the PS3 and 360 are just about tied when it comes to Guitar Hero and the 360 is easily the best choice when it comes to Rock Band when it comes down to total content available.
Honestly though, no matter which system you pick, it's gonna be a lot of fun.