Di maria is a very talented player but for every great game he has he has two where hes simply awful,as of now i dont think hes better than Nani at all the difference between the two is that while DiMaria plays almost every game Nani plays once every month and therefor Di Maria has more chances to show is talent and improve than Nani has.
For Me if Benfica gets a good offer for Di MAria they should sell him now that hes hot without thinking twice,he can become a great player in the future but at the present Benfica has other players like Fabio Coentrao that can do everything he does and with more consistency and also there is talks about Simao coming back next season,so if a big offer comes i think Benfica will and should sell him cause the club needs some money and we have players that can replace him.