Both (Maiden and Metallica) had great run of albums in the 80's, highly productive bands that toured alot but luckily with the reunion, Maiden has still been really strong and continued their quality from 2000 until today without making flops or mistakes.
Black Sabbath is also huge inspiration as they were among the first, if not even the first that really made heavy metal what it is. But they surely can't be discussed here as the catalogue is so separated with different singers and besides they aren't anymore as active as these 2 bands are.
I think there's not a real competition between Maiden and Metallica. Both are quite likely the last great heavy metal bands there are, sure there are younger bands out there but once these 2 will no longer be touring or releasing music, there's nothing left to replace them.
Metallica is sure more acknowledged and more widely known probably, but Maiden has more been true to their roots and so in some sense I don't think they have sold as much as their soul or become quite that commercial as Metallica. I think both are really huge, but Maiden is in some ways still regarded higher among the metal fans. I feel that the metal community actually respects Maiden much more. Here in Finland both are always in the news whenever they release new album or when they are doing some shows here.
I think Metallica's last truly great album is Load. Everything released after that has been patchy with some good songs but also lesser songs.