So I've attached a spreadsheet I use when analyzing mortgage rates and comparing a 30 yr vs. a 15 yr or whatever else you want to look at. It's not perfect and please facepunch me if there are any errors, I just thought it could be a good resource for anyone interested.

A few notes:

1) Put your numbers in the gray areas, the rest will calculate itself.

2) Columns A-E encompasses one mortgage (30 yr, or your current one) and columns G-K encompasses a different mortgage (15 yr, or a new mortgage).

3) Columns M-O calculate the difference in total payment, interest, and principal. P is cumulative principal savings.

4) Columns Q-S calculate your investment return on the cash savings from a refi, or from going 30 yr to 15 yr.

5) This assumes an 8% ROR on any invested cash, but you can change that assumption in the gray cell in that area of the calculation.

6) Column T is your cumulative Savings (Loss) by choosing the second mortgage.

7) In this scenario I used Diane C's mortgage balance and the rates I know I could attain in my current market.

If your still with me here, you should note that in row 80, column T, the momentum shifts. For the first 5 years the 15 yr mortgage is winning. Then the 30 yr starts to slowly pull back. In row 135 (10 years from now) the 30 yr mortgage starts to pull ahead into positive territory, and at the end of 15 years the 30 yr wins by $25K in net worth. Beyond that I'm not sure the formulas and math are correct, but here's the point of this exercise:

If you run the numbers and you plan to stay in your home for only 5-10 years, get a 15 year mortgage. If you plan to stay there longer, and you don't mind carrying debt like Diane and I (and ~25% of the forum members), then you likely should get a 30 year mortgage and invest the cash flow difference.