I would consider Priest more the trad metal band, and Maiden more the original power metal band. Priest are more traditional heavy metal, and have fun songs with an underlying heavy metal theme. Maiden have the more serious story telling approach, which makes them quite different. You wouldn't hear Maiden sing a song like Hot Rockin'...likewise Priest wouldn't likely tackle an epic like the Rime. Both are great in a different way.
Maiden have been more consistent throughout their career, and more often have the more consistent albums, but when Priest are at their best they are just as good as Maiden at their best. Again, it is hard, and kind of silly to compare them, because the song writing styles are so completely different.