I think in the USA Metallica is bigger band than Maiden by a fair margin. In America Metallica has the mainstream appeal since Black Album and today I feel their 80's albums are much more popular than 80's Maiden albums, while Maiden never had the mainstream appeal and they never sold a huge amount of their albums.
Outside of the US, it's a different thing, Maiden is popular as much as Metallica. In the UK they are definitely bigger than Metallica.
Even if Metallica is more popular band, they are not beloved and respected band as Maiden by metal fans.