I went with Iron Maiden, but this is how I feel about it:
I think that Maiden has a more solid discography all in all, despite albums like No Prayer for the Dying, Fear of the Dark and Virtual XI, I do think that Maiden has higher lowpoints then Metallica has.
However, I do think that Metallica's best albums, Master of Puppets, Ride the Lightning and And Justice For All (for me) are better then Iron Maiden's best albums, Powerslave, A Matter of Life and Death and Seventh Son of a Seventh Son (for me).
So yeah, Metallica has higher points, but Maiden has a much more solid/better discography overall.