If you're thinking 1 v 1, you're playing the wrong game, unless you're in a practice messing around with your buddies. The question is, how can you help the team destroy the enemy base the best?
Akali is a badass late game assassin once you get some items (Early if you can pull it off, but generally late). Much more so than twitch. However...
Twitch can eat a whole team in a team battle if things are setup right. He can also move about the map pretty well. Oracles can find you, yes, but he can start attacking people barely in their sight range to use it which means they aren't going to see it coming anyway.
As for twitch vs akali? I have yet to die to a twitch 1 v1, is it possible? Probably. It just hasn't happened.
I say decide your play style. If you have a lot of patience, twitch, if you have a mentality you just want to faceroll late game, akali.