Just thought that I would update this to answer my own question since I did take this route this past summer.
I asked several locals and park rangers in the area and they all agreed that Duck Lake Road was the better and safer route for biking from Babb to Browning, Montana. While there aren't any services on this route, there is also very little traffic. Several people mentioned how terrible Rt. 89 was with narrow shoulders and many curves without good sight lines. Duck Lake Road had great sight lines so you and the traffic could see each other a long ways off.
This route also provided me with a great introduction to the Big Sky of Montana and a nice view of the Rockies disappearing behind me. There are some climbs and descents on this route, but it's nothing compared to Going-to-the-Sun Road.
I liked this route so much that I think Adventure Cycling should offer this as an option for those who want to visit the Many Glacier area but do not want to venture up into Canada when riding the Northern Tier route.