Eli, if you're in the Amherst area you're probably thinking primarily about Routes 91 and 90 (the Mass Pike) both roads are illegal for bike riding and well patrolled by state police, who will probably not be too cool with your not having an ID with you if you make that choice. While I understand your desire to cut some time and ride on these roads you'll probably get little support from MassBike or other advocacy groups in your area.
I ride cross state frequently and if I could get on the Mass Pike and ride west or east my life would be easier- the interstate is, indeed, smoother, more graded climbs and direct but in reality Central and Western Mass. have some of the best cycling roads I've ever ridden. Once you're in your area Route 9 is very ridable, relatively direct, hillier but not bad. And route 20 is quite nice once your west of Springfield, also direct (runs parallel to the pike) and well graded.
Routes 47/116/66/112- I ride all of those and while the hills (esp on the east to west rides) can be tough if I were in that much of a hurry I guess I'd take a car. And I'm talking as much about riding recreationally as transportationally.
do you have a Rubel Central Mass Bike Map? The winding, hilly roads of the Amherst area are gorgeous. And believe me when I'm trying to get from Newton to Becket I'd love to do it in the 121 miles it would take me on the Mass Pike as opposed to the 140 +/- on the hilly back roads. At those times I can sometimes fail to appreciate the charm but I do think that ultimately one of the advantages of a bike over a car is that it demands that we sometimes slow down and smell the roses- something an interstate may not always provide.