I tried D3D and OGL on my Riva TNT2 which is now in the trash pending new computer.
On D3D, before it stopped working with newer builds that were released Q4 of 2005, the game did look prettier, and rendered better on screen than with OGL. Ambient factor regardless.
It's hard to explain the difference. It had a lot to do with the edges being drawn nicely in D3D while always sort of... *shifting* in OGL. OGL was stable though, rendered exactly like retail (when I say rendered, I mean behaved exactly as retail did, of course, not with the added graphical functions) D3D crashed with anything near-complex on screen. But it did look prettier.
I ended up using OGL though, no crashings and better FPS was far preferable to a more polished look in the game, considering the ancientness of the card.
At any rate, sounds like it'd be a good idea to drop D3D alltogether. No hassle with the end-users, no hassle with the coders. Simplification = good.
Just my 2 cents.