Ignore Dx10. On Dx11 You have something called "feature levels" that let you run on Dx10 and Dx9 hardware (the Dx9 level is a bit more limitied compared to original Dx9). If you want to write a 2D game or a relatively simple 3D game, I would go for Dx9. Otherwise Dx11.
I think I'll go DX10 cuz I don't want DX9, it is kinda old like I see it, and I can't run any f*cking sampals from the SDK.