Given current interest rates (much less than your mortgage rate!!) you should pay it off on a monthly basis as the interest calculated on the mortgage is monthly. Waiting means you're paying interest on the amount you've not taken off the mortgage whilst not gaining any valuable interest by having it in savings.
The only benefit to having it in savings would be as a safety net should you not have any other savings available for emergencies. That way, should you have an emergency, the savings would mitigate likely short term loan or credit card interest rates which are higher than that on the principle of your mortgage.