I prefer Nicko, who's just awesome... however there are places (like the whole Hallowed Be Thy Name song) where I actually prefer Clive... yes, I think Clive did Hallowed better, so what?
However, much as I love Nicko, this question really reminds me of the Theater's "Mike Portnoy x Mike Mangini" or Tull's "Clive Bunker x Barriemore Barlow" dilemma ... that is, I really like them both and the choice is difficult.
Nicko has the advantage of having played on much more material than Clive, though. That's definitely important, too.