Whenever you cant decide between two players, see which one you can get cheaper, and buy him.
Douglas Costa is the better player, but as you said he isn't really a winger (As in he crosses the ball, he's an IF).
I got him in the 1st season of my Arsenal save and he won PFA player of the Year, helping me to a Double. He wasn't as good in my second season, but he had to play rotate alot with the Ox and Isco. He still did quite well, getting 5 goals in the Champs' league and set up the winner in the Final!
So if you only want a crosser of the ball, get Greizmann. If you want an Inside-forward or don't mind then get Costa!
EDIT: Costa is also extremely versatile, so if you get injuries elsewhere he can cover for you!
went with Douglas Costa, looks a lot better and he cost 14.75m and Griezmann would of cost 21m