Perlite is a glassy, silicate, volcanic rock with a pearly luster and
many minute concentric cracks.
Vermiculite is "any of a number of hydrous silicate minerals resulting
usually from alterations of mica and occurring in tiny, leafy scales that
expand greatly when heated".
My understanding is that perlite is as inert as "puffed" glass. It has
no nutritive value to plants, and has no effect on pH. Vermiculite, on
the other hand, is mildly alkaline, is pH buffering, and does provide
some mineral nutrient value to plants.
I mostly use Vermiculite for mexican pings, and perlite/peat for a less
soggy general cp mix.
-- Rick