get dave's bike and throw some street slicks on it. loosing the knobbies for the street slicks will make a bigger difference than dropping 3lbs on the bike.
I commute on my MTB, but it's for the most part single track/dirt bike paths.
If I had a predominantly street commute I'd go for more of a street setup.
I got one of these: www.bigshotbikes.com
thinking i'd commute on it and ended up only really using it for around town. Like Dave said, pedaling into a head wind, with a back pack, up a hill, sucks with only 1 BIG gear.
my commute is 45min to an hour each way as well.