The biggest difference for me is McClair only had a couple of seasons with us, whereas McAllister was more of a Motherwell "product". When you saw either of them play, however, there was no doubting they were destined for greater things. (McClair single-handedly beating the OF in a week is still one of my highlghts of following this team).
There was a great article on McClair in The Scotsman a few weeks ago (even underboyleheating might approve of the gambling site he endorses - 50 % of proifts go back to supporters). He has turned into a bit of a hipster, though.