My suggestion would be a drop bar bike- either cyclocross, sport-road, or touring- and have the shop install bar-top levers if they are not standard on the bike (should run maybe $30.) This gives you the comfort level of a flat bar in a nod to your MTB background, but you still have the drops there for when you inevitably find you like riding fast on the roads, and start extending your evening commute or going for long rides on weekends.