The whole goal is to pay off the mortgage before retirement. What is the best way to go? It comes down to where you put the extra money each month. Let's take a look at the two options:
Option 1 - Save and Invest
This method would suggest that it makes more sense to take your extra money each month and invest the money into an investment plan. The notion behind this is that the investments will make more money and grow faster. Thus, you would accumulate a lump sum quicker to apply to and pay off the remaining mortgage balance. In theory and on paper, the logic makes sense.
Option 2 - Make consistent extra payments
The other option would be to make extra payments towards the mortgage either monthly or annually thus paying it down quicker. The whole idea would be to determine how much more each month that you had to pay extra to pay off the mortgage by the intended target date.
Which one is better?
Personally, I like the idea of paying the mortgage down through extra payments and not utilizing investments. In a perfect world where markets don't run into trouble, the theory of leveraging investment earnings makes sense. However, the worst case scenario is that you get close to the time that you need your investments and you sustain a large loss that takes years to get back.
Plus you forego the temptation to use that investment account for something other than paying off the mortgage.
After you account for fees, commissions, taxes, etc. the additional amount from investment earnings in my mind doesn't compensate for the market risk you take. At the end of the day, you want some aspects about your retirement to be a sure thing. Having the mortgage paid off is one of those sure things. Option 2 is about the math and the math is as sure of a thing you can get.