Invest Cash Flow or Put Towards Principle

[Levi Crabtree]

I have two rental properties. Both are getting just over 12% cash on cash return. Should I put the cash flow towards the principle of the loans to help pay off sooner (have 30 years on both), or should I reinvest the cash flow into my stock market mutual funds?  I have an emergency fund for both that equal 3 months mortgage payments for both.  Just not sure if it's smarter to put the cash flow into the stock market or put towards loan principles.  I'd like to know what others are doing and what has the greater benefit over time.