To become a test driver of Porsche, i'd imagine you need to be at the plant where the Porsches are designed, and manufactured. You'd then need to be an experienced racing driver too. And no, street racing experience doesn't qualify. :)
As for the 'best' Porsche, you will get different answers from everyone, for different reasons. Both the Cayman and 911's are very nice vehicles but they are on different levels. The 911 is the flagship of the Porsche lineup, while the Cayman is the new affordable sports coupe (along with the Boxster). I'm sure the base model 911 is faster than the base model Cayman, but then there are other things to consider such as handling, reliability, cost, resale value, etc. Seeing as the 911 has lasted this long as one of the world's most premier sports cars, i'd imagine we know for a fact the 911 is a good vehicle. Now we just need to give the Cayman some time to see if it lives up to the Porsche name.
Source(s):
Porsches in the family for my whole life, soon to be member of PCA