Is the BlackHawk free floated. If not, I would probably lean towards the Magpul from and ergonomics standpoint. If the BH is free float, I might lean that way only because you could add whatever acc. you wanted with out adding weight and stress to the barrel. If both are not floated and we are talking apples to apples, I like the magpul. Feels good, allows for the additon of rail sections for a few acc. and so on. Also probably considerably lighter than the quad rail.
Just my opinions.