I hate vs threads because each band is unique in their own way.  I actually love both of these bands equally for much different reasons so I will chime in here.  I LOVE the guitars in Priest, I love the look and the feel of the band. I know Rob has great screams but for me it is the angst and melodic tones that capture my attention most. I have met the guys a couple times and they have been gracious gentlemen.  I know they love America.  Glenn's guitar playing mesmerizes me.

My main reason for loving Maiden is Steve Harris.  His songwriting and bass really grab my attention here.  Bruce is a solid singer who has so much energy live - he is pure joy to watch on stage.  His off stage activities are also quite impressive, too impressive to not like the guy.  I could not tell you if they are gracious to their fans as I have never met them .  From what I understand they do not love the United States as much as Priest does.  Maiden seem to have a cult like following - sort of like the Grateful Dead.

Both bands have intelligent lyrics.  I truly appreciate that.  They all have great musicians, just different styles. 

** Special note:  I admire Glenn Tipton and Steve Harris very much.

So, there is my comparison.  Not very technical but it is what I see and how I feel.  Love them both for MUCH different reasons.