Difficult to pin down when to use perlite and when to use vermiculite,
because I will only use perlite nowadays. Vermiculite is OK inasmuch
as it provides some aeration to the compost, holds on to nutrients and
is water retentive. However, I find it much too water-retentive and
its aerating properties are limited when compared with perlite. It is
far easier for a vermiculite based compost to become soggy and stale.
As to its ability to hold on to nutrients, I tend to use composts
which contain a certain amount of loam, which is far more reactive in
terms of ion-exchange. So no great advantage there.

Used alone, perlite is the better medium for rooting cuttings and
encourages far more vigorous root growth from the outset. The
particle size and shape is such that it promotes faster rooting
compared with vermiculite and being both chemically inert and
pH-neutral, it can be used for all plants. Not so vermiculite.
Perlite is water-retentive to a degree, but does not hold such
quantities as to become completely saturated since its highly porous
nature allow for almost equal amounts of air and moisture to be
retained. Again, the particle shape and size prevent 'panning' within
a compost as it ages. I used to use a small amount of vermiculite to
cover seeds until I discovered that crushed perlite gave better
results.

I suppose its a matter of choice based upon one's own growing
techniques. However, having used both in commercial and domestic
situations, there is no doubt in my mind that perlite is the better
medium .... for me at least.

BTW both are acceptable to the Soil Association and therefore
compatible with organic gardening techniques.

Dave Poole
Torquay, Coastal South Devon UK
Winter min -2°C. Summer max 34°C.
Growing season: March - November