It will show on your credit report as "paid and closed" in good standing, so you might as well liberate yourself and pay the fucker off. You may have a *slight* credit advantage by paying over time, but nothing that will compare to the money you're throwing down the toilet at 15%. To put it another way, the fact that you got a loan and paid it off quickly is a plus for your credit history. If you paid it off a little slower it *could* be a little more of a plus, but not much.

FWIW, I have a ~810 credit rating and I have never let any debt go full term, including mortgage, cars, CC, etc.

:edit: only reason I can see not to pay it off is if you have some better use for the money, like a business investment or something.