I've got a nearly identical commute (mileage, climbing), and I have two bikes (data from Garmin recorded in SportTracks):
1) 1988 Schwinn Voyageur, added rear rack, fenders, usually (but not always) 6lb. laptop in a trunk bag and two panniers with food, jacket, tubes, etc... probably in the 40 lb range. Morning ride is lighter traffic, net 100' uphill, ranging from 1:06 to 1:17, average across all rides is 13.0mph Afternoon ride is sometimes a little longer (range 16 to 17 mi.) and net 100' downhill , ranging from 1:02 to 1:19, aggregate average is 13.4
2) road bike (either 2000 Fuji Team aluminum 18lbs bare or 1983 Trek 760 steel 21lbs bare), carrying nothing to a light backpack (3-5 lbs). Morning is 1:00 to 1:10 (14.4mph avg). Afternoon is about the same.
I'm usually between 235 and 245 lbs.
seems like the biggest factor is hills, the road bike option doesn't have the mph dip on the morning (net climb).