15 yr vs 30 yr with extra principal payments

If you just plan to make the extra principal payments on the 30yr, then yes the only benefit over a 15yr would be flexibility. But that flexibility comes with the cost of a higher interest rate and more total interest paid over the 15 year period. However, instead of making the extra payments on a 30yr, another option is to make regular payments but invest the difference between what you would pay on a 15yr. This would be the most beneficial scenario given you can find investments with an expected return greater than the interest rate on the mortgage.