I don't know what the added sales tax percentage will be in your area, if buying new from a dealer, but if you are talking a $3100 + Tax difference between a new unit and the used 2008, thats a fair chunk of change. When you say "the guy wants $13,500" , I'm assuming you don't have to pay any tax over an above this.
80 hrs on the used unit is virtually a "new" tractor, and to my knowledge, there really aren't any big differences (if any) between the used and the new one. As long as the used one hasn't been "abused", and you should pretty much be able to tell just by looking for dents, scuffs, scrapes and the like, how well the guy cared for his equipment. I wouldn't have a problem with it with only 80hrs, on the clock. Nobody can even tell what year it is simply by looking at it.
The big consideration will be, if you need to borrow the dough for the used one, what will the cost of your loan be. If you add that cost to the $13,500 the seller wants, how will that stack up to the "new" unit.
I'll bet you'll be well over the 17 grand mark or possiby quite a bit more, if you start adding some of those "must have" extras, once you get down to the short strokes. Make sure you get an "out-the door", delivered price. Sometimes once you get down to the nitty gritty, the off the cuff quote of $16,600 will have a tendancy to inflate to a lot more than you originally planned on, what with those nice little extras like freight/administration charges/delivery etc. Things they don't even mention until it comes time to write up the sales contract.
Whatever route you take, good luck.
REV