If you love spinning, then road biking is most like spinning. Having both a road and mountain bike I would say both can be equally intense and fun. I think with a road bike you can jump on, roll out of the house for an hour and get a really great workout. For a great mountain bike ride sometimes you have to drive out to the best trails, so there is more preparation and set up involved, but once you are in the back country seeing deer, foxes, and coyotes it can be pretty awesome.
But my advice is to ask your spin class friends what they are riding, and how often. If you have some friends to ride with either road or mountain, that is always the most fun. So if your class is mostly roadies, get a road bike. Mostly mountain, then a mountain bike. Most cyclists always like introducing new people to the sport.
I personally don't think there is a 100% best choice, both are good.