The Sawmill Parkway is an awesome, very pretty road! The upside is a continuous undulation of ups and downs and curves - it's really a lot of fun but you get sucked in to keeping up with traffic. 60mph pulling a trailer on a narrow road can be exciting if a stoplight suddenly comes up.
The down side is it's narrow, so it's really hard to pass anyone when you're pulling a trailer AND IIRC there are 3 or 4 stop lights on the length of it to watch out for.
I don't know how much faster it is than going farther east to pick up I-684 off of I-287 and north that way - Maybe 20 minutes? The big gain for the trip was not taking I-81 all the way to Scranton to pick up I-84.
The quickest route from SC to MA looks like it's I-95 all the way (Google thinks so, too), but the traffic in the 5 big cities you pass through always slows you down, especially across that terrible road surface of the GWB and Cross Bronx Expressway. Coming up I-81 to I-84 is almost exactly 100 miles longer, BUT overall it's the same and often less time than the coastal route and can be a lot quicker with bad coastal weather or a few more traffic slowdowns due to accidents or any sports events causing heavy traffic in DC/Baltimore, Philly, NJ, around NYC or Hartford. Those are the killers.
Of course, if you get an accident on I-81 just before an exit you can have a ten+ mile backup in a matter of minutes. Always remember to bring dog treats.