Priest came first and helped solidify what has become known as heavy metal. Maiden is one of the best-loved bands ever in the genre.
Which group do I prefer?
Cannot answer. They both have been mainstays in my musical diet for the better part of 35 years.
Motorhead had a huge part to play in solidifying what has become heavy metal as well. Bomber, Ace of Spades and Iron Fist were pro-to thrash albums. Of course, Sabbath are the grandfathers of metal no question about that. We can also talk about bands like Budgie, Thin Lizzy, Purple, King Crimson, 70's Scorpions, Uriah Heep, Wishbone Ash, Pentagram, Accept, UFO, ACDC, Blue Oyster Cult, Van Halen, and so many others, but yeah, there was something evil and sinister about albums like Sad Wings of Destiny, Sin After Sin, Stained Class and Hell Bent For Leather that took metal into a new direction and had a huge influence on Maiden, Saxon, Diamon Head, Raven, Tygers of Pan Tang, Witchfynde, Tank, Blitzkrieg, Angel Witch, Tokyo Blade, early Def Leppard and many others (all before Metallica's 1982 EP).