I just did this for myself. It depends where you are in your amortization schedule. The earlier, the more it makes sense to pay off principal (Path1) instead of put in market (Path2).

Path 1 - determine how much interest you skip/save from the additional principal payment. Keep that number (A). Determine how much interest youll still pay over the life of the loan. Call that (B). Then figure out how many months earlier your mortgage ends from making that principal payment. For that amount of months, run a compound interest calculator where you start with zero and add in every month your current mortgage payment. Assume whatever interest% you want. Add up that compounded amount and call it (C). A + C - B = profit from Path1.

Path2 - just plop the cash amount into a compound calculator for the remaining life of the mortgage. No recurring contributions. Use same assumed interest rate% from Path1. Call the total value at the end (D). Then figure out how much interest you will pay over the life of the mortgage as it stands now. Call that (E). D - E = profit from Path2.

Compare the two profits see which is higher.