Invest in Stock or Pay Down Debt?

I'm getting ready to refinance my mortgage. I never thought I'd take out an adjustable rate mortgage, but a 7/1 is 3.625% at ING. And since seven years is all I have left to pay off my existing loan, in my case that ridiculously low rate will be for the life of the loan.

It will also knock down my required monthly payment by two-thirds. What if I made the minimum payment and invested the rest?

On the up side, I could really get ahead if I did well in the market. Joel Greenblatt's Magic Formula has worked very well for the past few years. If it continued to do well, in just seven years I'd be ahead tens of thousands of dollars.

On the down side, those returns aren't guaranteed. If the market tanked, I could end up behind. Historically, there have been decades when stocks went nowhere. Plus, I'd only pay down my mortgage by a few thousand dollars, and still be making mortgage payments in my seventies. That's depressing. Having refinanced to a 15-year loan seven years ago, I've paid down my mortgage by almost half. That feels great.

I'll have a mortgage burning party in 2018.

I have another loan that's about to be paid off in a few months: my car loan. I'll start taking the monthly car loan payment and buy stock.