It's somewhat of an apples vs. oranges comparison, so the answer to "Is scenario A the same as getting a 7.3% return on the $500?" is "no."

Take a slightly different example, where one invests $100 in A that returns 10% for 6 years, vs. investing $105 in B that returns 11% for 6 years:

A: $100 invested, $177.16 returned

B: $105 invested, $196.39 returned

With B, $5 more was invested and $19.23 more was returned. That does *not* mean the extra $5 returned 25.2%. Do you see where your A vs. B comparison is similar?

Compare a 30 year mortgage vs. a 15 year mortgage, *both with the same interest rate*, and the extra money available with the 30 year mortgage will need to be invested at...exactly the same rate as the mortgage rate...to be able to pay the principal after 15 years.

Your larger question of "which option is better?", however, seems well calculated. Going to the lower interest rate on the entire principal means you need something much better than the 4.375% on the relatively small amount you can invest by keeping the lower monthly payments.