Both can pack a mean melee punch.
I'd say Orks have less units that DON'T work in melee, the only units that come to mind being Mek Guns and Grots, whereas Nids have Termagants, Neurogaunts, most of their shooty monsters (who aren't AWFUL, but definitely aren't particularly good for the points in melee!).
Either one can make a good melee list, though.