Help with mortgage payments please

Hi, Newbie here so please be kind!

My mortgage is up for renewal and I have decided to stay with my current lender for various reason and go for a fixed rate.

We currently pay £1003 per month. We have an overseas investment valued around £100,000 which will eventually be completed and sold to pay a chunk off the mortgage. But it is unlikely that that would happen over the next 5 years.

Mortgage is made up of two parts and term is 22 years which we are really keen to reduce.

Part 1 £86,000 @ 1.74% above base rate until the end of the term, we have decided to keep this as is, so not involved in the remortgage.

Part 2 £100,000, we have been offered 3.19% for two years which overall is a saving of £97.07 on what we are currently paying, or 3.69% for five years a saving of £71.12 per month.

My question is what would people opt for two years or five years.

And (this is the bit I can't get my head round.) how would overpayments work. So say we went for the 2 year deal but overpaid back in by £97.07, or took the 5 year deal and overpaid by £71.12.

Any advice would be appreciated. Need to make a decision in the next couple of days.