My favorite commuter tire is the Marathon Slick and I perfer as wide as I can fit. However my commute is only 8.5miles (~13.5k) on urban streets. For your distance a 28mm Gatorskin is probably a better choice. I don't like running narrower as I find it uncomfortable and prone to rim damage. Ofcourse my roads are rough and I weigh about 220 pounds (100kg).
Craig