You can't compare these two because they are in different price ranges. The 390x has higher price:performance. The 980ti is just higher performance if you don't mind dropping $650 on a GPU.
So I should change my DPI to 800-1000 everytime I launch the game in Synapse and change it back everytime I close the game? Is there an easier way to do this? I play at 2200 DPI and 2 Sensitivity