I feel that there is some confusion here regarding what a mountain bike is.
These two bikes for example;
Same brand, both 2012 bikes. They look pretty much the same. I get people telling me all the time that both are mountain bike. That is completely incorrect. One is a hybrid, the second is a cross-county (race) mountain bike.
The top one is a hybrid in it's purest form; a road and mountain bike fused together. Relaxed geometry. Tires are not too thin, and not too wide. Small travel in the suspension to cushion rides. Disc brakes for great stopping power, which are not affected by weather, therefore perfect if you do commuting all-year round. You can use it for commuting, leisure rides, light long-distance and doing some light trail rides; such as cottage-country up north and whatnot.
Again, NOT a mountain bike. As much as it looks like one.
The bottom picture IS a mountain bike. Longer travel up front, wider tires for more grip, bigger rotors for more stopping power in the trails, and a heavier/stiffer bike compared to the hybrid above.
I understand the confusion, but even then; nothing wrong with using the bottom one for the city. I have a friend that does. He doesn't want to buy 2 bikes, so ended up with a 29er MTB so he can use for both trail and the city.
Pick the bikes based on what your goals are, and again; TEST RIDE. It must feel right. No matter how sexy it is, or how good the price is.