I use Shimano M120 shoes for spin classes. The spin bikes at my club are all SPD cleats only, and I did not want to mess with changing out the cleats on my road shoes each time I spin.
I picked the M120 because they are pretty cheap ($90 at REI) but overall seem to be good shoes. They have rated well in reviews and other evaluations I have read online. The soles are not quite as stiff as my SIDIs, but stiff enough for a spin class. On the whole, I like the M120--they are not SIDI comfortable by any means, but pretty cheap and good enough for spin.
I don't think it matters whether you get mountain or road shoes. Most of the people in my class have either road shoes or those odd mountain/walking shoes with the recessed SPD cleats.
I would just get something comfortable in your price range. They should be comfortable (otherwise you won't wear them), but they don't have to be perfect (spin classes aren't that long anyway).