I commute in Chicago 365. Chicago is flat and you can use whatever bike you like - fixed/single speed, mtg, hybrid, cross, road, european city, cruiser or frankenbike. The choice is yours. Your are over thinking the situation. I commute on a road bike (nice one) 3 seasons and switch to a hybrid for the winter. Only things to consider is where you are going to park it during the work day. Theft is a major concern so don't get a bike for your commute that you aren't afraid to have ripped off (no $2000 road bikes unless they sit in your workspace or secured parking). If you are worried about being passed, get over it. It ain't the bike , its the motor.