Don't take my word from it, but i've heard that Directx 10 is kinda, well... not used much. Directx 9 or 11 are the most used ones. Why you choose Directx 10?
I believe that DX9 is supported still simply because older cards still cling to it but aren't up to DX10, DX10 is for newer cards on XP, DX11 ships with Win7 only.
DX10's advantage over DX9 is more shader flexibility. And a little performance. But that's about it. I really cannot claim to know more on the subject difference between the two.