Sales tax is collected by state and local governments, each locality handles it differently. Unless you can cross state lines you can't control it, so ask your dealer about it before you start any negotiating. You may have to pay the sales tax up-front and then request a refund when you file your state taxes.
As for which tractor is better, how many acres do you farm and how many hours will you put an the tractor in a year? Some people around here consider 5 acres a big farm, others in this same area consider 640 acres a small farm.