Vi brings more utility, Zed brings straight (but potentially very high) physical damage. Vi's a better jungler in my opinion, as her ganks are much MUCH more powerful (especially post 6, her ganks are practically as good as Nautilus) and doesn't require as much gold to be effective. An underfed Zed is painfully weak, as is pretty much every assassin.
You certainly can jungle Zed, but don't use him as a ganking jungler; use him more as a farming/control/counter - jungler, since he is so ridiculously fast at clearing and can escape easily if needed.
My personal vote is for Vi, but Zed certainly has his uses and isn't a bad champion.