AusBonsai

Could anyone advise which is best. I have half a bag of the coarse Purlite, which I thought was pumice expanded by heat, so I wanted to continue to use this up, before I purchased anything else.

Pumice is much harder than perlite. I think perlite is made from glass?
You can use it but it is a little fragile and very light.
You can also mix it with vermiculite for layering.

Perlite is great stuff, yes it is light and you can crush it between fingertips but it has benefits such as ready supply of o2 to root systems as well as transferring moisture across its surface.
Depending on the size you use, it's water holding capacity will vary.
Larger particles = 25% WHC approx.
Smaller particles = 30-35% WHC approx.