Another vote for a used bike. Single speed or hybrid won't make that big a difference for a 6 mile commute. But, if you start riding more for fun the extra gears MIGHT come in handy. However, being that you are in grad school, you may find yourself turning into a hipster doofus in which case you will need the single speed. (Preferably with non-matching neon colored rims and bullhorn handlebars.) At that point the messenger bag will be a vital accessory.
Seriously though, I would advise against buying a new bike at this point. You will quickly learn what you really like or don't like and then can spend the big cash on your next bike. Get something that fits and ride it like you stole it--you know, like the punk that steals it from you will.