1: depends on where its from. if its from peru, i say peruvian torch. (supposed to be more potent then sam pedro)
2: depends on supplier. id go with powdered (as long as its fresh)
3: as for preparation, id tell ya to go to the store, get 3 lemons, get a pot that isnt made out of aluminum. put enough water in the pot so that you can put all the contents of the powder into it without becoming a sold sludge. add 3 lemons, and stir. do NOT bring this to a boil. boiling cactus is a BAD idea. it will quickly boil over.
bring the cacti to a simmer, and stir continuously for about 2-3 hours. take the sludge/snot and let it cool down.
final step (and the hardest) drink the sour barky tasting snot and ENJOY!