Elise shuts down everybody. Vi turns team fights into a 4v5 by slamming the ADC. Fiora snowballs way harder than either of those, but has less favourable matchups. Learn Fiora, and she is way, way better than people make out.
I'd pick up Fiora, definitely.
Vastly underrated. Her lane phase isn't bad at all, and she's a hyper carry. Takes a little time to master, but you won't regret it, at all.
Don't bother getting Fiora, since you won't have the porno skin.
I'd say get Vi, because she's really likable and does very well in the current meta. She even has a few good matchups in the top lane. As stated before, she's great in the jungle. By all means, play her top lane if you can win it because she can do so much with the gold she'll get from farming.
Did I mention she has an awesome theme song?
Don't worry Summoner. Sona_Buvelle will protect you. http://i.imgur.com/xx0hSNV.png