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system
extra cash - go towards mortgage, or stock?
user
it's six of one a half dozen of another.   investing the cash is a little more risky.  you know exactly what you'll get by paying down your mortgage.  if you have a solid emergency fund it's probably most advisable to pay down your mortgage.   if your mortgage is 3% and your investment makes 3.5% you're talking about a taxable gain of 0.5% on the additional cash.  is that worth it to you? sure, the s&p has been on a tear but remember, past results are not a guarantee of future performance.
assistant