I was wondering the same, so did a bit of looking around.
Telegraph review says:
"The drum kit is a far more solid offering, and it has far more responsive pads and cymbals than its Guitar Hero: World Tour counterpart."
Eurogamer Review:
"If you want a nice new plastic Strat with a sunburst faceplate, metal pretend tuning pegs and a much-improved tap bar with little grooves to guide your fingers, or a new metal drumkit with a detachable control panel that's much more solid, reliable and aesthetically pleasing than World Tour's, the only way to get them is to buy a Band Hero kit."
So they seem to be better, just a pity they're not sold on their own yet.