I prefer an upright position over the drops for a winter commuter as you get good visibility and easier to deal with the clothes you are wearing. A lot of folks commute on MTBs for exactly that reason. Why do you think your mtb won't be a good commuter esp. since you have discs on it. I'd take your $600 budget and put it to work on a pair of continental winter contact II tires and a heck of a good light system. If you don't like the bike as a commuter after a winter, you can move the tires and lighting system over to a new bike next winter.