If you don't physically have the ram on the video card, for DX11, then use DX9.
If you do have sufficient ram on the card, most would go with the DX11.

I believe the break point was 1.25GB or less, for a five boxer, meant DX9 was superior.
But as a five-box, if you have more than 1.25GB, then DX11 is probably the way to go.
Not sure on a ten-box, but I'd assume ten is twice the resources of five... meaning 2.50GB or less would force you to use DX9.