OpenGL or Direct3D? I have a 980TI. also, it flickers when I go into fullscreen.

[Helios]

OpenGL is recommended for nvidia users because their OGL implementation actually works and lets you take advantage of some of the really cool things we do for better performance and more accurate emulation.

So for you, unless the game's page on the wiki recommends otherwise, use OpenGL.

AMD and Intel users should use Direct3D.