openGL is by far better, but you also have to take into consideration the card that you are using also. if you have a voodoo card (esp. an older one, i can't remember if the v5500 supports openGL) the only way you're going to run openGL is with a wrapper, which will suck your will to live. the performance will be worse than just running in D3D.
if you are using an nvidia card your openGL performance will rule D3D with a vengeance.
the thing is games ship with 2 api's either D3D and GLide or D3D and openGL. D3D is that cross platform thing that they throw in so that no matter what card you have, you can run the game.
since D3D is a "jack of all trades" it is also a "master of none."
<font color=red>booyah, grandma, booyah..</font color=red>