I'm new to this board, but not to cruising or Tahiti. We've done it twice with Princess.
That being said, for your first cruise in the region, I would recommend the TP - the region is expensive to begin with. The PG is all-inclusive, with the exception of some excursions, so it's more expensive off the bat. Princess will be less, but you will have to pay for more things (like specialty coffees, soda, alcohol, etc). The ship is small and give you a chance to really get to know your cruisemates and the crew, but at the same time it's not so small that you can't find a little spot all to yourself and relax.
We enjoyed the ship, the itinerary and the folks we met along the way. For snorkeling, diving and juse scenery, it's hard to beat Tahiti