First of all, paying off your mortgage early vs. investing the difference is not an apples to apples comparison.  I don't know of any investments that provide a guaranteed return = to the mortgage interest rate in question. 

If you are comfortable with taking on the risk, you might be better off stretching that mortgage to 30 years and investing the surplus cash into a side fund.  

Additionally, this could provide liquidity along the way to access "just in case."  

However, there is something to be said for my clients that are mortgage and debt free in retirement.  Therefore, those qualitative factors should be considered AS WELL AS the quantitative factors.