Riding conditions were my biggest factor. My commute is about the same as yours in terms of distance. The last leg of my return trip is quite a bit of hill climbing. I made the trip for 2 years on a knobby tired mtb and it wasn't too bad in the Spring/Fall when temps were moderate, but when it was upper 90's/100's, it was brutal. I switched up to a road bike with drops, but a little bit wider of a tire than most road bikes and couldn't be happier. I didn't see a huge change in commute time due to the lights/stops. But I noticed a huge difference in effort.