The X Factor is a masterpiece compared to No Prayer For The Dying. Unpopular opinion?
It wouldn't surprise me if Maiden had sold more than Metallica taking out the US.
It is known that Metallica is a band that is huge in the US and have sold a great deal of the records there whereas Maiden is much more distributed worldwide.
Besides, maybe Metallica also plays bigger venus worldwide, but it doesn't mean they sold more records.
For example in Argentina most people I know who go to Maiden venues are real fans who buy the records.
Metallica has lots of casual fans who won't buy it.
Maiden fans are special and very loyal worldwide. It is one of the few bands that sells more with each new record even though there are less sales everytime because of internet in the music business.