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Paying Off Principal of Home vs. Investing In Mutual Fund
Paying off the debt is low-risk, low-reward.  You're effectively guaranteed a 4% return.  If you buy a mutual fund, you're going to have to take some risk to have a decent chance of getting better than 4% and change return in the long run, which probably means a fund that invests primarily in stocks.  Buying a stock mutual fund is high-risk, high reward, especially when you're in significant debt.  On the other hand, 4% and change is very low-interest.  If you wanted to buy stocks on margin, financing stock investments directly with debt, you'd pay a heck of a lot more.  Bottom line:  It comes down to your personal risk tolerance.