This separation is a useful distinction, although it is often difficult to make in practice.

It's a useful distinction when thinking about King's personal life.

The most useful general distinction to make in this context is between religion and spirituality.

Later, however, he no longer felt this to be a useful distinction, and preferred the term flap in all cases.

A number of useful distinctions have been made by philosophers in the treatment of value.

But there is a useful distinction to be made in noting that the cancer comes in two main forms.

However, later on, he no longer felt this was a useful distinction to make, and preferred to use the word flap in all cases.

So if there ever was a rule about 'which' it's no longer a useful distinction in English as actually used.

There is a difference between a rule and a useful distinction.

Nonetheless, the list below offers a number of useful distinctions between possible research designs.