[JohnN]

Paying off the mortgage is the "sure" thing, basically a fixed return investment.
Investing it will give you whatever the market brings, put it in China? I don't know..
I don't see spectacular returns given the multitude of problems (social security, Iraz, borders, trade deficit, budget deficit, etc)

I like (as noted above) being debt free and spreading my risk into real estate, which is different than stocks.

My intent, and I'm much like you, near/at retirement, is to minimize debt.
Good luck.