DX11 mode in GSDx will work (currently) on any DX10 card since, again, it doesn't contain any DX11 specific shaders or calls.
So... should I just stick with 10? or will there still be games where 9 would be better?
D3D9 is still better for some games but it's not many, basically use D3D10 and if you find a problem in some game try changing to D3D9 it might help and might not.
DX9 is fast
DX10 is faster and looks better
DX11 has yet to be reveiled for its true power