Both do more or less the same job, but one lasts longer than the other, and I can't remember which way round it is...

I prefer Perlite as it keeps the compost open. I find Vermiculite clogs. Remember to water Perlite before diving into the bag, it is very dusty.

Generally, perlite holds more air than water, while vermiculite holds more water than air. For plants that like fast drainage and drought, such as succulents, add perlite. For plants that like to stay moist, such as ferns, add vermiculite.

Vermiculite is also useful for covering small seeds in seed trays. It lets the light through and keeps moisture around the seeds. Perlite is good for mixing with compost for cuttings or for things like Lavender or Thymes, anything that likes good drainage as Tugbrethil says.