I sincerely doubt both will come. They could, but than we will have to rotate heavily in every single game since there is no way Messi, Suarez, Dembele, Coutinho and Griezmann can play at the same time.
RW/RM is taken by Dembele, Central postion or second striker is taken by Messi and CF by Suarez. So if Suarez stays there is only room in LW position in 4-2-3-1 or AM in 4-3-3 system or LM in 4-4-2 system and Griezmann in not suitable for any of those positions.
This is the reason I would prefer Coutinho since he can play LM, LW and AM while Griezmann can play only as striker or winger. Griezmann is also left footed so IMHO he would be better on RW and than you have to move Demebele to the left. I like both of them but if Suarez stays and regains his form I have no doubt that Coutinho is better choice for us right now. Biggest problem with Coutinho is that he is injury prone.
That being said if Suarez continues to play like this and if we sell him I would go for both of them in a heartbeat.