By your own admission you clearly secured a repayment level which is several hundred below your actual maximum. 
The difference of paying interest only and paying the payoff amount is about $200, I plan on paying at least that, if not more, each month. I just like the fact that I don't have to the full amount if I don't want to. Since the interest rates are the same and I know I'm going to refinance asap to get rid of the high interest on the 20% loan, theres no reason not to give myself some room. Theres obviously is some risks: what if interest rates skyrocket? what if theres some disaster that causes housing prices to plummet? I only have to wait 6 months after my house is built to refinance and get rid of the 20%, and I feel the benifits are great enough and the time is short enough to get the no interest. And if something does happen where I can't refinance, well I'll still be paying the same as I would if I got the "safe" loan since I plan on paying the extra couple hundred each month anyways. Also, I doubt I'll live there for more then 5 years, which means if interest rates skyrocket, I'll be out of there before my loan is up anyways. And if prices plummet....well, then I'm screwed no matter what I do. If both happen, then I pack up and run to mexico. Actually, if housing prices start to drop, then I'll sell before it gets too bad, rent until the market stabilizes, then buy again. Being an appraiser I always have a close watch on what the market is doing.