Interest rates are at an historical low. They are guaranteed to increase over the next 17 years. They are almost guaranteed to increase over the 6 years left in your current mortgage payment schedule.
If they stay where they are for a while, the interest offered on savings could decrease further.
If you extend your mortgage, you will end up paying a lot more than 300 a month in 10 years time.
If I was in your shoes, I would pay off the mortgage as quickly as possible and then
think about saving, based on the economic climate and interest rates at the time.