Clive was a very solid, tight drummer and the better fast drummer. I'd say his hi-hat work was better than Nicko's or at least he could play a fast pattern more steady. Compare Hallowed and Wrathchild. Nicko hits his hi-has twice as slow (not sixteenths notes).
Clive's work in Genghis Khan, but also Phantom and Transylvania (well: all Di'Anno era stuff but these examples are among the ones that stand out) is amazing.
Nicko approached most material of this era in his own manner, but that's also because I think he is not able to play Clive's way. That said, Clive could probably not play the difficult stuff as in e.g. Sea of Madness or LOTLDR, or fast bass drum work that we know from various songs in the catalogue.
So when we purely focus on elements of the kit: I'd say Nicko is the ride cymbal and foot specialist, and Clive the hi-hat.
But when you judge them overal, there's more to say of course. This is what Maiden guys said about Clive, also comparing with Nicko:
Bruce Dickinson considered him to be "...the best drummer the band ever had. That's not taking anything away from Nicko. Technically, Nicko's probably a far more competent drummer than Clive. It's just that Clive had this incredible feel, and you can't learn that, and I regret that he wasn't given more time to try and sort himself out." Iron Maiden guitarist Adrian Smith also praised his drumming ability. "Clive was a great drummer, an Ian Paice-type drummer, steady and solid with a nice feel to everything."