I want neither but if I had to choose I'd go Ibra.
Cavani has been next to useless for 3 years. For Uruguay and for PSG.
He's this decade's Dario Hubner.
Edit - Actually I take that back, Hubner was better.
Sorry Emmet but I have to say that that is absolutely not correct, although I know more about the Uruguay part than with PSG. I'll admit that even that isn't a lot but I do talk about football with Uruguans every day (they won't ever shut the fuck up about it and neither will I

)
Last season with PSG: 53 games, 31 goals
This season with PSG: 46 games 20 goals
2015 Uruguay 8 games 4 goals
2016 Uruguay 2 games 2 goals.
Plus he had to step up and carry Uruguay without Suarez, and outshone Luis in both of the games since his return- in terms of performance also in goals, with Suarez only getting 1 in the two games - and I think I remember that Ibra was injured this season? If so that means he stepped up to cover the difference there too.
Cavani is an elite striker and would improve any team. Us included.