I am a broke noob and can only get either one, I usually play akali and ezreal if that helps (ps not really good with ezreal but enjoying to play him). I feel that either one will suit my purpose of being an assassin kind of champion that can at least sustain and even though I do like kayn and his abilities, the amount of times I was able to counter his ganks as akali kind of made me feel distasteful for him but I think it's just the players that could be using him wrong. While talon I kinda like I don't feel he has that epic broke mobility compared to what kayn which makes kayn the superior at get aways and chasing his enemies down.
Let me know which one you think would be better to get and if you possibly can, list your points. I appreciate all the comments and feedback