Griezmann is better than Coutinho, and by a distance too. He's scored 25 goals this season, Coutinho has scored 39 5 seasons at Liverpool. Griezmann also has more assists than Coutinho. PC is a good player but he score or assist as often as he should.
Also, I'd be very surprised if Man United spend money on a striker with Rooney, Martial, Rashford etc. Also, they'll have to spend a serious amount of money to get someone, which prices them out of Griezmann.