I have always been able to print out a 25% off coupon with PreFlight...didn't matter if I was flying out of Logan or TF Green in RI. And the fact that they have a shuttle that comes right to the car, loads your luggage on and off you go, is wonderful. Econony at Logan doesn't offer that. We have had to park in the nether regions of that lot and lug our bags to the shuttle bus area. If I have to choose, I tend to go with PreFlight.
I'm in central Mass and have used Knight's transportation to get me to Logan on many occasions.