the problem with shopping only at local bike stores is that they will go out of their way to recommend that you buy only whatever they have in stock. for all the praises sung of the LBS, I must report that I have been underwhelmed by the disingenuous and even downright dishonest approach of many stores I've been to.
Although you may want to buy locally, I would recommend instead getting all the information online that you can before setting foot inside a store. In addition to the price/speed questions above, I would include the following:
* do you plan to frequently ride in the rain? (If so, you may want to consider a bike that doesn't use rim brakes.)
* Are there a number of intersections on your route, or do you find yourself needing to stop suddenly? (If so, you may want internal gears that let you shift while stopped.)
* How much of a maintenance guru do you want to be? (If "not much", consider internal gears/brakes as well as a chainless setup.)
* do you plan to ride at night a lot? (If so, you may want to consider a bike with a generator hub to power the lights.)
* is your commute "multi-modal" in that you mix it up with a bus/subway/train or drive partway? (If so, think seriously about a folding bike.)
good luck. there's also a good suggestion floating around to pick up a beater on craigslist and ride it for a few weeks - that'll help you get a sense for your route and which characteristics are important to you. I would've never known at the start that I wanted a bike with internal gears, internal brakes, and no chain--but I'm glad I waited and got one!