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.

Full Build Zed can exceed 500 AD with his passive

Vi is an excellent jungler right now. It just depends on what you want to do. They're both pretty good at taking down the enemy carry, but Vi thrives a little better in the middle of combat. Personally, I would go with Vi just because I like her attitude and yelling "You have been chosen!" whenever i initiate onto their carry.

/

zed

Zed.

Much more fun and rewarding.

Well you really have to ask yourself one important question...
Have you ever really wanted to be a total rebel flipping tables on the enemy?

Hmm, I would like to owned my enemy.