I've done both of those routes. I still take 91 99% of the time. In the early morning hours or late at night RT 7 can be quicker, however with all of the small towns, slow cars, and fluctuating speed limits The Doctor mentioned, it might only end up 10-15 minutes faster.
He hit the 'annoyance' factor on the head. Definitely have to obey the speed limits, too. Plenty of town police officers take notice of those who are speeding, similar to the Rt30 drive to Stratton. However, it is very scenic when the sun is coming up in the morning, but you won't get the benefit of the route at night.
I typically take 91 because you can ignore the 55mph speed limit in most places and go with the flow of traffic at 65-70 to make up the difference in time that your map is showing.