It's impossible to advise without knowing your personal circumstances, but personally I've opted for mortgage overpayments for the following reasons:
A) I am lucky enough to have a defined benefit pension which even without additional contributions should provide enough for a reasonably comfortable retirement.
B) DH is going to be a SAHD so all the burden of paying the bills is on my shoulders. Whilst my job is pretty safe and the sick pay is generous, I want to minimise our outgoings just in case something really terrible happens and it is also building up an asset which we would have available in case we ever needed to sell. DH is also naturally anxious and I suspect it might be easier to convince him we can afford a second child if our bills are low.
C) By reducing the outstanding debt we can get a lower monthly payment when we come to renew (nb I prefer this to reducing the term because we still have the option of overpaying 10% of the outstanding debt per year which gives us flexibility that we wouldn't have if we reduced the term). The lower bills mean that we might be able to afford for me to reduce my hours when my mat leave finishes.
D) Who knows what's going to happen with Brexit and more generally interest rates in the future? In theory we would be fine even if they shot up to 10%, but it wouldn't be easy and so the less outstanding the better.
E) We've got over 23 years left on the mortgage and I know that overpaying at this stage makes a massive difference because so much of the monthly mortgage payment goes towards interest (it is almost half in my case!) rather than capital.
F) This reason makes less sense but I just hate being in debt. I check my app frequently to see how much is left on the mortgage and enjoy making the overpayments and seeing both the outstanding debt and the wasted interest money reduce each month. I can still remember when I finished paying off my student loans (which incidentally I didn't overpay) and the mixture of relief and satisfaction that I felt.
In my case as a higher rate tax payer I definitely suspect I'd be better off on paper putting money towards the pension, but the short term gains and the peace of mind for me and DH are more important to me at the moment.