Gray,

If I were you, I'd use my road bike on the trainer.

* You can get used to the different positions on the road bike -- on the hoods, on the tops, in the drops.
* Your road bike and mountain bike may fit you differently, so you may use your muscles a bit differently on each. (Why is this so hard for me to articulate?) Use the muscles you're going to use when you're riding on the road!
* As others have mentioned, using the road bike on the trainer doesn't stress it that much.
* The knobby tire on your mountain bike will drive you nuts on the trainer!

- Melissa

PS Regardless of which bike you use, you're going to get a huge cardio benefit. You'll be a force to be reckoned with come springtime!