Appears you understand quite well.

Let's play what-if?:

a) $75,000 loan, 5.1228% over 10 years. Payment = $800/mo (oddball interest rate to make round payment). Extra $1,200/mo invested at 7% compounded monthly.

After 10 years, the loan is paid (including $21,000 interest) and the investment has grown to $207,700.

b) $75,000 loan, 5.1228% over 10 years. Payment = $800/mo, with $1,200/mo to pay down principal. After loan paid, $2,000/mo invested at 7% compounded monthly.

Loan paid in 41 months (including $6,900 interest). After 79 months investing (and 80 months return from $100 left over after loan payoff), the investment has grown to $200,100.

But if the investment return is 4% instead of 7%, a) grows to $177,700 while b) is $180,500.

All the above just reinforces that you "should" put your money toward the item paying (or charging) the highest interest rate. But, as you state, the market returns are "unsure" - and then there's the whole philosophical thing about being debt-free or not worrying about being debt-free.

Anyway, back to the original point: it appears you understand the issues and trade-offs so the choice really is up to you. Either way you will be better than spending the extra money on mocha double lattes or similar.