If we look at the two older versions of the players, I too think Cavani fits United much better. The thing with Zlatan, even in his old version, is that he can win matches even playing in weaker teams. He is better with good players around him, but he doesn’t need them in the same way as Cavani. You can hoof the ball and he will suck it in and create something. I have watched Milan quite a lot and it is incredible how he create.
That's it in a nutshell. We had Zlatan at the right time as we were a terrible side and that's exactly what we were needing. As we've seen, it took some time for the link up and understanding between Cavani and others to kick in. He wasn't going to transform us all by himself, he is a system/team-player, not a one man team.
All in all, I wouldn't swap them around in either direction, both are better than the other in the sides each had the opportunity to play in.
That said, I was always baffled we didn't try sign Cavani earlier. Chelsea kept being linked as well. He was made for the Premier League and would have suited both LVG (pressing, workrate) and Mourinho (proper centreforward, warrior, no emotional management nonsense). Despite spending years shunted on the wings Cavani is PSGs all time record goalscorer, domestically and in Europe, not Zlatan. Still, you get some here saying Zlatan can only compare to Messi and Ronaldo, go figure.