I tend to think of 6% as a very low interest rate and if you can borrow at a low interest rate and invest and earn a higher one, then I think you should. (When I was just getting started with work, home buying, and investments in the early to mid 1980's, co-workers were getting home loans at 13.25%. My husband and I bought our first home with the help of a guy who was a first in our town: a mortgage loan broker. He snagged a 30-year fixed rate loan of 8.25%.)

You are guaranteed the 6% return and the 10% return is not guaranteed. I tend to be less risk averse than the average person or at least than many of the financial advisors that I've encountered and so would not necessarily speed through my mortgage at such a low rate.

If you have other investments, the guaranteed 6% will lower your overall risk while allowing you to enjoy some market returns (hopefully). If the $100 represents all of your extra cash and you have an either/or situation (either pay off the mortgage quickly or invest ), it would make sense to me to invest some of the money. You don't want to be a 50-year old (assuming you bought a home at 25 years and paid it off early) with no investment experience.