Both of these pay out if you die during the term. The difference is the shape of the money. Ordinary life cover pays a single lump sum. Family income benefit pays a monthly amount to your family for the rest of the term instead. Same purpose, very different feel, and the right one depends on what the money is actually for.
The lump sum
A lump sum is simple and flexible. It can clear the mortgage, and whatever is left is the family's to use as they see fit. It suits a clear, large, one-off need, a mortgage being the obvious one. The catch is that a large sum handed to a grieving family at the worst moment of their lives is a lot to manage well, and it has to last.
The monthly income
Family income benefit replaces the wage rather than the balance sheet. Instead of a single payment, it delivers, say, two thousand pounds a month until the term ends. For a family used to living on a monthly income, that is far easier to handle than a large cheque, because it arrives the way a salary did and covers the same everyday costs.
It is also usually cheaper. Because the total paid out falls as the term goes on, there being fewer months left with each year, the insurer's risk reduces and so does the premium. For a young family wanting the most protection for the least cost, that matters.
One protects the balance sheet. The other replaces the wage. Most families need a bit of both.
How to choose
Match the tool to the need. Protect the mortgage with a lump sum, because that is a single large debt that a single payment clears. Protect the day-to-day cost of raising a family with an income benefit, because that is a monthly need best met monthly. Most households are not choosing one or the other at all; they are using each for the job it does best, often in the same plan.
The mistake is buying only a lump sum because it is the version everyone has heard of, then discovering it was sized for the mortgage and left nothing for the fifteen years of ordinary living that follow. Work out both needs separately, and the split becomes obvious.