Depends to a certain extent on their personal circumstances.
When we had a regular, defined income we took out fixed rate for as long a term as we could get, in order to provide predictability. Quite possibly we lost out due to falling rates, but if rates had gone the other way, we would have been protected. I still say it was the right decision, even though it turned out more expensive.
Personally, I like security and predictability, so would tend to go for the 10-year deal.
However, 10 years is a long time. Family circumstances can change considerably in that period. You need to look at the penalties for early redemption, and also if you can move the loan should you want to move house.
Probably best to see if you can get some independednt advice, but no-one here is going to be able to predict interest rates over a 10-year timeframe.
Often the lender will guarantee you a rate for 10 years as the borrower (maybe 4%), At the same time they will offer a guaranteed rate (maybe 3.5%) to an investor. Each side is tied in with costly early redemption fees, so they have a guaranteed profit.