Have you considered putting slicks on your mountain bike? (slicks are skinny non-knobby tires that come in MTB sizes.) The great thing is trying this out won't cost much money, you can get a pair for about 30 bucks.

I use an older Trek 8000 mtb with slicks and a rack to commute 10 miles from Brooklyn to Manhattan and this works out just fine. I also own 3 road bikes (one a Davidson built in Seattle, try Elliott Bay Cycles down by Pike Place Market) and wouldn't even consider using one of those. 5 Miles on the BG isn't that long a ride unless you need to climb one of the big hills at the end once off the trail.