Overpay on the interest only or repayment element ?

injectionday

Hi.  I’ve been inspired to try to do what we can to make overpayments on the mortgage with a view to be able to pay it off a few years early.  We can initially afford to make regular overpayments of about £60 a month with the intention of then making extra payments when we can but I’m not sure if if would be best to make the overpayments to the interest only or repayment parts of the mortgage?

The figures are:
 £58k interest only with 6 years left.
£59k repayment with 13 years left
both at 2.29% interest

We have an endowment for the interest only element which will pay £55.8k if it grows at 3.3%

Any advice would be appreciated. Thanks