as said, alloy vs steel. the deore ones also had a plastic kind of cover that is easy to break, which was all part of the clamp on teh xt i think (?)
i was going to upgrade back in the day but i got put off by stories of the alloy clamp being fragile. dunno how true it was.
other than that the black/yellow always looked cooler than silver, but the caps are interchangable if you can work out how (they screw in from underneith.)