It kind of depends on what you are going to use it for. If you want a good all-around bike (races, riding around town, riding on trails, etc...), go with the mountain bike. A road bike is pretty limited as to where you can ride it. You will get a better bike for $150 than you will a road bike too. I'd probably go used, though, as you will get a much better bike for the money. You can always switch out the tires to a smoother tread for road races. You can do some races on that bike, then you can decide later when you have more cash if you want to fork out the money for a road bike as well. I have both, and the road bike does make a huge difference on the road, but it isn't practical to ride with my kids on the bike trail (they ride REALLY slow). If you're just doing it for fun, it really doesn't matter what kind of bike you have. You'll see everything.

at that price range, you'll be looking at kind of a klunker bike. If you plan to mostly road ride, maybe a used road bike would be fun, but like a $500 car, you can be buying a lot of problems if you don't know what you are doing--ask around and see if you canget some help with the purchase.