I'm late to this thread but here are my thoughts. One, for games like Overwatch you don't need much, a 1050 Ti will do fine. It'll even play more demanding games like GTA V and Battlefield 1 fairly well. On the other hand "maybe other games in the future" suggests you'll probably be playing new games that come out, games even more demanding than today's games. So I think the 1060 3gb is what you should get. Your i5 can use the extra performance of the better card.
One last thing, you should upgrade to 8gb if you want to play the more demanding games. It doesn't make a lot of sense to spend the money on a 1060 but run with 4gb system ram. For Overwatch and other esports type of games 4gb is alright, but then I'd advise getting something like the 1050 or 1050 Ti for that.