Personally I think he's best when switching wings periodically with another both-sided winger such as Ronaldo or Obertan. I think he's prone to growing a little stale given a significant run solely on one side of the pitch; if he cannot switch during games, i.e. when Giggsy or Valencia are fit, I'd personally like to see a formation in which he switches sides every three games or so, as doing so seems to give him an added freshness which makes his attacks more effective. I wouldn't particularly say though that he's better on one side than he is on the other. I think Nani gets the best out of Evra, so that's reason enough to play him on the left, whereas on the right he's more selfish and direct (in a good way), so that's a good reason to stick him on the right. He's a very, very good player.will you ever let it go?
Nani's is better on the right than on the left. Full stop.