You might as well be comparing Slayer and Cannibal Corpse. Both styles have their place and they can be enjoyed in their own right.

I prefer Frazetta too, but Vallejo is rightly recognised as one of the greats. He does his own thing. Not everyone can, or should try to, be Frazetta. That'd be pretty boring.

Or, like I once told a friend: you gotta stop comparing everyone to Bugs (Bunny).