For this game, opengl is probably a good choice.
There is no or little difference between the 3DFX and opengl versions from what I've seen and been told. Open GL offers more screen resolutions IIRC? But it might depend on what video card you are using as well... I've been using it on NVidia products.
There are definitely other 3dfx games where 3dFX mode unlocked certain aesthetic bells and whistles. Reflective floors better lighting and darkness, transparencies, higher resolutions (for its time), etc. King's Quest 8, Undying, Unreal, etc.
But I'm not sure what it does for this game?
One thing to keep in mind is that 3dfx was 16-bit card (well last generation Voodoo 5 was 32 bit I think, but very few games were made specifically for it in mind), and modern cards are 32bit or higher. So there are some definite color differences between the modes. Different set of textures even. D3D probably uses the higher color textures.
Nglide forces 32bit rendering, but generally speaking 16 bit textures are just forced into 16 bit.