this been discussed alot in FW900 thread, and IMO they are equal. BNC is what your paying for(quality), it doesnt matter whats at the other then, BNC>VGA is alot more popular then BNC>DVI, and will be easier to find at a good price.
Depending on whats on your graphics card vga or dvi; for example mine has 2 dvi ports but no vga, so i bought a dvi>bnc cable. This should be superior to buying a vga>bnc and putting a dvi>vga adapter on it. But if you have both it probably doesnt make a difference.
Thanks guys, thats what I wanted to know. My friends getting a fw900 and he has the option of VGA or DVI to bnc, so I wanted to find out before he bought the cables. I couldn't find the answer in the fw900 thread, must have not popped the right phrase in search.