When we bought initially and when we added on, we were offered two interest rates: the one-year financing (50% down) had a 4.75% (I think) rate, and we took that for two add-ons; the other rate was for 3,5,7 or 10 years and was 9.75% for all categories. Just to repeat, it was the same interest rate for all terms over 1 yr.
IMO, take the longest term you can to maximize financial flexibility, even if you only need to do it over a shorter term; the ten year deal minimizes the minimum due, allowing you to pre-pay in flush months and pay the minimum in lean ones. With our initial contract, we signed up for ten year financing, and pre-payed it in less than 2. But, if we had run into leaner times in that period, we would have been able to pay the lowest possible minimum bcs of the ten yr term. An added bonus is that you can pre-pay on your credit card, and get whatever premium your card pays; in our case we used the Disney Visa, and the rewards points are covering DD's annual pass this year.
Good luck with your calculations. I think tjkraz' breakdown above should be helpful.