Assumptions
1) The 15% you mention is your expected income tax bracket in retirement.
2) The mortgage interest rate is "low" (e.g., ~3% )
Put the money in the 403b.
Take $100 in salary. You can
1) Invest all $100 in the 403b, earn ~7% (historical average) then pay 15% when you withdraw principal and returns later.
2) Pay 28% tax now and save 3% on the $72 lower mortgage balance.