Nicko isn't really a flashy drummer and doesn't have the same creativity as Clive in the beats he comes up with (Gangland, 22 Acacia Avenue, and Transylvania come to mind for me), but is still more consistent and skilled. Nicko is a much stronger drummer overall in my mind, with better breaks, fills, and technical prowess.
It's as simple as this: Clive had better feel, whenever that guy played you could just hear the emotion and see how much fun he was having during live performances. Nicko on the other hand, while still having some feel he's much more technical in his playing and wants perfection every time. (Hence the reason why he got so mad when he "fucking missed it")
As above, both awesome for their times. But I just loved Clive's sharpness and precision, and the swing he brought to those three albums. He was solid and tight, but the swing was something that, for me, no other metal band has had. Ghengis Khan FFS!
I love Nicko, his creativity (Eagles Dare) and some of his fills (across the Powerslave album) are immense. But he's never been the best at keeping a tight rhythm going. He could race away with a song 25 years ago, and can get left a touch behind the rest of the band these days.
The band was definitely more steady with Clive, but I think Nicko is the right drummer for the band. The interplay between his drumming and Steve's driving basslines have been the foundation of their signature sound ever since. It's just so much more exciting to listen to, even though most musicians would argue that it is not exactly considered good musicianship to let the tempos fluctuate by the amounts that they do.
They both work equally well for their respective tenures with Maiden. Clive sounds great on the early stuff, Nicko has definitely been the man for the job on everything after that. I would lean towards Nicko on this, but maybe that's because he's such a central feature of the biggest body of their work, including what most of us probably consider their best.
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.
Such a hard comparison considering the difference in time served. What could Clive have done? Who knows but its hard to argue with Nickos legacy and them tom rolls and that Right Foot. That said, I love Clives performances live and recorded, he has such a swing that fits the Maiden of his time. So, overall, Terry Bozzio shades it by a hair lip. Just.