Hard to argue with a dude has Eddie in his Avatar, but IMO, Judas Priest has had more mainstream success than Maiden, and the HOF being what it is, puts Priest in ahead of Maiden. Priest has sold over 50 million albums, and they had more radio airplay than Maiden.
IMO, Maiden is more of a cult band like Rush or Tool, and there is nothing wrong with that. After typing that, I see they have sold over 200 million albums, so maybe I should just shut up.
Maiden did not become really successful until Dickinson joined in 1982. Priest started in 1969.
I did not mean this to come off as a slap against Maiden. They deserve to be in too. They are like Coke/Pepsi of British metal bands. Both solid, and many fans have a strong preference.
BTW, Firepower is a pretty solid album.