Let's further assume that you need a $100,000 mortgage and pay a higher rate (e.g. 9.5%) for a 30-year mortage instead of a lower rate (e.g. 9%) for a 15-year mortgage.
For the 30-year mortgage at 9.5% you need 360 monthly payments of $840.85 each;
For the 15-year mortgage at 9.0% you need 180 monthly payments of $1,014.26 each.
You need additional $1,014.26 - 840.85 = $173.41 after-tax dollar to pay for the 15-year mortgage. So you have $173.41/0.65 = $266.78 pre-tax dollar less per month to invest in 403(b).
So the question was: Should I put this $266.78 pre-tax dollar in 403(b) or take the $173.41 after-tax dollar and send it to my mortgage banker in exchange for a 15-year?
PAYING OFF THE MORTGAGE ROUTE
After 15 years, you owe nothing from mortgage banker, you have no assets in 403(b). Your cash asset balance is 0.
403b ROUTE
After 15 years, you still owe the mortgage banker $80,526.04 (not much less than $100,000), but your $266.78 monthly contribution accumulate at 8%/12 monthly rate you have cash balance of $92,317.66. However you only have $92,317.66*.65 = $60,006.48 in your account. Your net cash worth at that time: $60,006.48 - $80,526.04 = -$20,519.55.
The net effect is that you just lost your children's one year college education fee (at the time you exactly needed).
SOME MORE POINTS:
A dollar earned in you own mortgage is a sure dollar, while a dollar earned in your 403(b) at 8% may not be so sure (you might do better or worse, the sure thing is that it is more risky).
Generally, it make sense to have more debt at a high inflation period, but not make much sense at current situation.
What about a peace of mind by having less debt?
So the conclusion is obvious: choose a 15-year mortgage. No wonder 15-year mortgage is so popular NOW!
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>p.s. I'd be delighted if you found a flaw in my argument.
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The flaw is that you ignored the marginal dollar's ability to bring down the mortgage interest rate. What you owed is compounding at a larger base and what you have is compounding at a smaller base (not speed or rate).