I rarely use perlite now.
It is good for plants that require a very open textured compost. Trouble is with open textured composts they tend to dry out quicker, perlite helps to alleviate this problem.
I still use vermiculite but not as a compost improver/texture adjustor I find sand or agricultural grit does equally as well certainly for most veg, and is generally much cheaper.
I use it for seed that require light to germinate because it does a number of things e.g.
1) It lets light through
2) It holds moisture so keeps the surface sown seed from drying out
3) It holds the seed firmly in place once it germinates, rather than lifting the tap root up in the air before it can penetrate the compost.
Regarding quality this depends upon what you are going to use it for.
If it is for a compost aeriator then the medium to rough is best. Fine is used for what I descibed above.
But don't waste you money on the fine you will find that if you seive the medium/rough you will get enough fines to cover all the seeds you are likely wanting to cover!
I hope that helps!
edit; have look at pictures 23-28 here;
http://www.thegardenersalmanac.co.uk/Data/Seed%20sowing/Seed%20sowing%20in%20quarter%20trays/seed%20sowing%20in%20quarter%20trays.html