some people think metal is an attitude, some think its how much distortion there is, some think its how raw or brutal or fast, some want to say old metal "isnt really metal", some want to say even old hard rock was metal
I seperate hard rock from metal in more of a music theory and rythmic way. The type of groove thats layed down and the attack of the guitars. I'm going to have a hard time explaing this but its something I can just hear.... Metal uses lots of palm mute pedaling riffs, most times using all 7 notes of the mode or scale, sometimes mode changes, the rythmns are typically steady pounding, you know.... bang your head. Rock mostly utilized the 5 note pentatonic scales with passing note from the blues scales, the rhythmn pattern of the riffs is more like a shuffle or has a boogie feel, I dont know..... more of a sway than a bang your head.
Priest did a good job of combining both but you could still always hear their roots which reach back pretty far. I wouldnt deny them metal status, that would be crazy. They may very well have been the ultimate transition band because they had a good feel for both and were there in the middle of the transition period. But I would still say and if my memory serves me correctly Maiden was considered heavier, like a step further. By the time Piece of Mind came Maiden was the latest thing, heavier, darker, more brutal. Alot more than that was going on around that time but thats my take on the Priest/Maiden differences.
If you read a bit about it, apparently there was some pressure on Preist for "falling behind" in the brutal department especially with thrash coming into popularity. Seems like Maiden never ventured even the slightest toward thrash but just stayed their path. You can hear Preist adding some thrashy ideas later on.
Doesnt matter, both bands are unto themselves but Maiden was heavy metal through and through, Priest was drawing on a few styles.
Also looking at wikipedia last night it appears through the 1980 to 1990 recording period both bands have sold about equally album for album in the US. Nothing overrated about good sales....
http://en.wikipedia.org/wiki/Judas_Priest_discography
http://en.wikipedia.org/wiki/Iron_Maiden_discography