both have advantages but aggressive fish have so much more personality. i have a bit of both in my tank, i've got some shoaling lampeye killfish and serpae tetras which are all peaceful and i've got them with 2 kribs and a firemouth (semi aggressive) and to be honest the peaceful fish are pretty dull. they just don't really do anything. also they all look exactly the same, but with the others you can tell them apart.
agressive fish is definately the way forward. i would love to get some oscars or green terrors