Assuming $200,000 mortgage, with 1% interest spread...

That is a bigger spread than is realistic IMO, but I think the important thing is that you wouldn't pay of the last 15 years of the mortgage in a lump sum. I ran the numbers for how much cash you would have in a savings account investing the difference every month for 30 years, or investing the full amount beginning 15 years in.

$200,000

15yr@2.75% = $1357.24/month

30yr@3.75% = $926.23/month

Difference = $431.01/month

Invest $431.01/month for 30 years. Compare that to Investing $1357.24/month for

15 years. Compare the end value with different rates of return.

30 yr total - 15 yr total = difference

4%: $299,142 - $334,003 = ($34,861)

5%: $358,711 - $362,775 = ($4,064)

6%: $432,956 - $394,710 = $38,246

7%: $525,819 - $430,193 = $95,626

8%: $642,359 - $469,656 = $172,703

9%: $789,068 - $513,587 = $275,481

@7%, MMM assumption, that averages to $265.62/month for 30 years.

So the 15-year wins if your rate of return is 5% or less. However, we are talking about a 30 year average. Since the historical rate of return over the last century has been around 10%/year, I'll take that bet.