Agreed. PoM has overall a higher number of quality songs throughout than Powerslave.
But...production wise, style-wise, finesse-wise (of where the band was in their 'arc'), I'd recon Powerslave was them at their peak.
And yes, I prefer Clive Burr to Nicko, just like I will die defending Les Binks being the best drummer Priest ever had!
Both Clive and Les had a certain swing, almost jazzy/funky feel to their grooves.
Oh, and soooo stupid "Total Eclipse" wasn't included on the American and European pressings of NotB. That's a great fuckin' song. At 2m39s into it, Maiden basically starts doing Mercyful Fate.

Regarding the Maiden vs. Priest stuff, here's my 2 cts, and given that I've played in multiple tribute bands of each, it might be worth 2.5ct

) :
I'd say that Maiden has a higher base level of quality in their songs, but rarely deviates from that. Say on a scale from 1 to 10 (10 being best), Maiden's base level sits around 7 or 8, where occasionally a song is a 6 or a 9.
When it comes to Priest, I'd recon to say that the base level of quality is a tad lower, but they deviate more. Priest has some absolute horse shit songs that were either just plain shit or just trying too hard to become anthems (United, Red White and Blue, Take on the World).
But against those, there are pure gems that potentially surpass some of the (sometimes better) ones from Maiden.
I mean, if we're comparing the sort of slow, clean, moody stuff, say "Strange World" vs. "Dreamer Deceiver", Priest wins that one easily for instance.
And Screaming for Vengeance, Stained Class, The Ripper, Devil's Child, Painkiller, Freewheel Burning, Ram it Down....all fuckin' bangers.
...but, that being said, I enjoy playing Maiden more usually. Except when I crank Unleashed in the East, I usually play along completely, with vigor.
And how can you not love a band that has 3 songs about BJ's on one album?!