I love both and ultimately don't feel the need to pick a side, but I have more respect for Maiden. They might have released a mediocre album here and there and some of their recent albums are a little bloated, but they were never the shameless sellouts that Priest were. Priest seemed willing to change their sound in any way, shape or form to fit the trend.
Priest have some awful albums. Point of Entry, Turbo (save a few songs), Ram it Down (save a few songs), Jugulator, whatever the one after Jugulator is called. That's five legitimately bad albums out of 16. And it's not like Nostradamus is an album that's worth listening to start to finish.