David Lang wrote:

Blair asked
Screwfix show two types of cavity fixings as above
Are there different uses for each?

Probably just cost per 100. I guess the metal ones are better, but if you
are going to use a lot the nylon ones are cheaper.

Never quite understood trying to save money on consumables. Wood glue is a
good example, Titebond 2 versus PVA. Titebond is double the price, but glue
is about .01% of the cost of a project, so why not buy the best?

Because in many instances it may make no difference.
Knowing when it might is of course the key.