It all depends on your attitude to, and appetite for risk. Paying off your mortgage gives you a guaranteed return equivalent to the mortgage interest rate, reduced slightly by the effect of TRS.
If you believe that the shares you are buying will give you a higher return than this after all taxes and charges, then shares are the way to go. But you have to accept that the value of shares can go down as well as up, or even be wiped out altogether.
Incidentally, you mention that you won't have much of a pension. There are usually various tax-efficient ways of improving your pension, which could be a third option. Some ("self directed" or "self administered" pensions) even allow you to continue buying shares, but with tax relief associated with a pension.