Yikes, bankrate says rates spiked 1/8-1/4 yesterday because of a good jobs report and that subprime bailout plan.
Regarding the 15 year plan, lets assume two different fixed 200K mortgages:
5.5 30 year - $1135
5.0 15 year - $1581
Which seems about correct to me, its usually only a half of a point difference but you might be able to find larger differences elsewhere. Now lets assume your goal is to pay it off in 15 years.
With the 15 year, you just make payments as planned. With the 30 year, add just the difference in monthly payments. $1135 + 446 and the loan gets paid off in 15 years and 10 months. So that half a percentage point buys you 10 months off the term. Or, alternatively you could pay an extra $54/mo and make it the even 15 years. 15 years of $54 is certainly some money, but not a lot. And it buys you some flexibility. If you lost your job, or had more expenses then expected you always have the option of switching to the 30 year plan or even just taking a break from it for a few months.
Its just one way to go. But I personally don't feel like the loss of flexibility is worth the saved money.
DBL pointed out you could invest the extra for a better return. Personally, at this point I'm going to sock my extra into HYS since I'm pretty risk averse and need to build back up our emergency fund, which was depleted from putting a down payment on the house. If things go to plan and its still around later on...I can also use it as a lump sum to pay off the mortgage.