Well, both of them are wild animals, so both are aggressive, but probably a Grizzly bear. Black bears are more curious than anything, and will only attack when threatened or if you look like prey. Grizzly bears will attack on the same circumstances, but fed bears [ bears who have been fed by humans ] can become agitated if you don't feed them, causing them to attack. Both can kill, but I'd say that Grizzly bears will dish out more damage, more dependent on their size, anyways.
