Dont stint on good commuter tyres, get ones with the best puncture protection you can find. If you stick with your MTB, then a high pressure 1.25 to 1.5" slick is ideal. If you decide to go with a 700c road bike, then make sure it has clearance for something practical like a 28mm width.
There is some pretty std commuter advice: carry a spare inner+tools, let the bike carry your luggage. If you ride in the wet use fenders and in the dark, use lights. Wear helmet and gloves in case of a spill and some predal retension (clipless or toe-clips) will add efficiency on a longer ride.