First off neither of those two bands are the best metal band, that honor belongs to Black Sabbath and it ain't even close if you ask me.
Now, I voted for Iron Maiden for the simple fact that I think Killers is a fucking monster of an album. It was, in my opinion, the best fusion of punk/hard rock/early heavy metal I've ever heard and every song on there is great. Their later material with Dickinson would never touch that album although they had some good songs here and there (well, The Number of the Beast was solid all around as was Powerslave, arguably).
Judas Priest has a couple of great albums, too -- British Steel, Stained Class (my favorite), Screaming for Vengeance and Painkiller -- but none can equal Killers.
Here's a question for you guys, what's the most underrated song from both artists? My response will follow.