Scott Trench wrote a blog article on the subject with a spreadsheet you can download. What are you going to do with the excess cashflow from a 30yr mortgage? Are you going to spend it, or invest it? In Scott's blog he assumes investing it into an S&P 500 index fund. Under his default settings on the spreadsheet the 30 year mtg leads to greater wealth. The nice thing about the spreadsheet is you can run your own numbers and see what happens. I just did a 30 yr refinance and put my numbers in the spreadsheet. My interest is higher than his default. I assume the stock market is not going to perform as well as his defaults based upon the current dividend yield of the S&P 500 and the overall return over the last 5-10 years. Under my situation it looks like 30 year mtg comes out slightly ahead of 15 year net worth, but cash flow from year 15 until 30 blows away the 30 year mtg until year 30.
A 30 year mtg will probably come with a higher interest rate, but you are buying the flexibility to pay the lower rate. You can always pay it like a 15 year mortgage with the extra going to principle until you want or need to pay the 30 year rate.