Tough call.
Maiden 80-88 all strong albums. Very consistent. Killers and Piece of Mind are brilliant
Metallica 83-88 all strong albums. I'd take MOP and Justice over Killers and POM though.
I can't say I'm crazy about either past 88. They're both way past their prime in terms of writing new material. They're essentially both just live acts.
As for Maiden, post-88, Fear of the Dark and No Prayer are dull as was the Blaze era. And as much as I liked Dance of Death on the first few listens, they just didnt' take any real chances with that album.
I'll give the post-88 edge slightly to Metallica. I like Load and about half of St. Anger. I think they did something different with those albums.
Really in 2005, I think they are both nostalgia acts, maybe more so Maiden because of the fact they only played songs from their 1st 4 albums.
I rather listen to the last Meshuggah album than either at the moment.....