Vi or Nocturne (I didn't know which Board to put this in)

I pretty much main jungle, but i only have the one champion im really good at (Amumu), so i wanted to get a new champion. My friend said i should get Vi, so i was saving up for her then Nocturne came on the free week and i played him and now i kinda like him, but i don't know which one i wanna buy.... So my question is, Who's better?
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