I've been playing along with both lately, it makes it all that harder to choose. There's a JP concert from 1983 that's so much fun, but Iron Maiden doesn't get as boring and has some cool spaced-out parts and long intricate parts. Glenn Tipton is a mother-fucker, one of the best ever, period, much better than KK, Adrian, or Dave as far as me learning the lead correctly. JP like Slayer used to list which guitar player played what solo on each song, I'll bet that was a weird decision meeting.
Slight edge to JP just because of Glenn.