Over all, this is a very tough call, as I have loved both of these bands for years, to say nothing of the influence they have had on the metal genre. Honestly, though, the way I think of it is like this:
I'll take Priest at their best over Maiden any day. As much as I love Maiden, the really good Priest albums, Like Painkiller, Defenders, and Stained Class, are far and away better then anything Maiden has ever done, IMO. However, Priest certainly has their WTF moments, and, sadly, they've had many of them. For every Painkiller, there is a Turbo; and for every DotF, there is a Demolition. While I prefer it when a band takes musical risks and tries something new with every album (which is a big part of why I like Dream Theater so much), that really only works if the end result is actually something good. Turbo is a perfect example of this: The title track is awesome, and if the whole album sounded that good, it would have been a fantastic record. But after that, it just becomes 80's glam 101, and while the songs aren't really bad per se, they aren't really anything special. Or take 2001's Demolition. I find it ironic that Priest actually managed to make a pretty decent album without Halford when they did Jugulator, and by and large, alot of the new things they were doing on that album worked. So they actually succeeded where Maiden and so many others had failed with a new vocalist, only to throw it all away with a lackluster follow up. Demolition was a mess of styles and creativity, as if Glenn and KK had no idea of how they wanted to "modernize" the classic Judas Priest sound, even though they had already attained that on Jugulator.
Iron Maiden, on the other hand, has never really changed that much over the past 35 years, and despite going through their ups and downs, it has worked for them. The biggest musical sin they committed was not picking a Dickenson wannabe when Bruce left the band (like Priest had done with Ripper, who actually could do the Halford material justice live). Other then that relatively minor stumble in the mid 90's, though, Maiden's career has been surprisingly solid, and for good reason. Like AC/DC, Maiden found a musical formula that works for them and their fans, and they stuck with it. As much as I love it when bands try something new, there is something to be said for sticking to your roots, and while I may not feel that Maiden has as many "classic" albums as Priest, I know I can pop in any Maiden record and enjoy it. Every musical genre needs a constant, something that is always there and never changes, and Iron Maiden is probably the biggest constant in metal (Motorhead is probably a close second). With Priest, its more hit or miss.
I guess what I am getting at here is that as much as I love Priest and respect them for what they've done for metal (and in all honesty, they do deserve more respect and credit for making metal what it is today then Maiden), taking their discographies as a whole, Maiden has been more consistent in the quality of music they put out, and as such, have an overall higher proportion of quality material. When the history of metal is written, I believe metalheads of the future will look back more favorably on Maiden then on Priest because of that unwavering devotion to their stylistic roots and relatively consistent artistic excellence. They will no doubt fondly look upon the the peaks of Priest's career as well, but they will find little to admire in the valleys that lie between them. So for that reason, I vote for Iron Maiden as the overall better band.