I didn't really buy songs with keys on PS3 for any particular reason. I just like Queen and The Doors and a few other bands, and their songs happened to have keys. If I knew the 360 keyboard would be cheaper than PS3's, I'd have bought the songs on 360 lol.
Also I'm a co-leader of a RB team so I play cross-console for more FBFC possibilities. Only cost me $30 extra since I already had a 360.