Definitely Mega-Dactyl. M-Beedrill is an offensive pivot.
I am pretty sure that the reason is his lack of powerful coverage stab-moves.
Sludge bomb and fell stinger are his two main stabs... i think.
Even with the base power effectively being 180 and 100 respectively, neither of these have the best coverage.
Mega-Aerodactyl has a number of decent moves, especially combined with tough claws.
Although, some of his more useful moves don't get that tough claws boost. (I'm lookin' at you, Earthquake)
Then again, i could be completely wrong.
I like Mega-Aerodactyl better, but it's mainly because I lack the competitive skill to use Mega-Beedrill effectively.