Nothing wrong with mountain bikes per se, it's the ones that are more eye candy oriented with cheap ass front and rear shocks. As pointed out previously, they a) won't last b) have crappy components that are pretty much disposable c) often come with wannabe offroad tires that suck for the street.

The sort of hybrid commuter bike gives you the best of both worlds - comfortable body position via the geometry, narrower tires, larger diameter wheels (in some cases) and gearing more appropriate for street riding than full on off road gearing. For example, both http://toronto.craigslist.org/bik/152057997.html or http://toronto.craigslist.org/bik/151993641.html are hybrids. The one has drop bars on but could be fitted for straight bars. Short answer: more comfortable body position, faster bike, more bang for the buck in terms of parts than the dual suspension bike you had in mind., Perhaps less theft appeal, although bike thieves aren't that discriminating in my experience.

good locks: In the U of T area, Urbane Cyclist http://www.ucycle.com, Bikes on Wheels http://bikesonwheels.ca/ are two off the top of my head. Check TBN's listing for something close to home if those don't suit http://tbn.ca/retailer/rtdb/at.htm

Expect to pay around $60-80 minimum for a half decent lock.