This is a tough one.
I’ve only seen Maiden live. I was a casual fan before that show. Now I worship every album from the debut to Seventh Son. Adrian Smith’s guitar work in particular on Somewhere in Time is iconic. Maiden definitely has the more consistent output. They wouldn’t release a subpar album until Dickinson left in the 90s.
As for Priest. The main thing I like is how much their sound changed over time. On paper it wouldn’t seem possible for the same band to release Stained Class and Painkiller. Painkiller also brings one of metal’s greatest what ifs. If Halford hadn’t left it would have been interesting to see if the band could top their greatest album. Priest also has the more enjoyable recent output in my opinion. Maiden’s more recent work can drag from “chorusitis” as I call it with songs being too long due to overly repetitive choruses. However I think British Steel is the most overrated “classic” metal album of all time. I vastly prefer albums like Sad Wings of Destiny, Stained Class and Painkiller. Tipton and Downing’s growth as guitarists is astounding. Painkiller is a hands down top 3 contender for greatest metal guitar solo. I don’t think I can pick one.