We have cruised on Renaissance R Four in Tahitiian waters twice in the last year, and will return to do it again this July. That should give you some indication as to as to whether we enjoyed the cruise. We absolutely loved the Ren experience.
As for the PG, we cannot give you much information; although we ran into some of their passengers on Raiatea on our last trip. We talked with some of them, and were asked how much our cruise cost. They were amazed at how reasonable Ren's prices were for what we got.
The itineraries are similar, but they spend less time at each venue and/or have fewer venues. For example, their package is only seven days versus ten days for Ren. Yes, they do have a diving/watercraft launching platform on the stern, and you get all the "free" booze you can drink; but in our estimation those things (and a few other intangible goodies) don't justify their much higher prices.
Check it out carefully, and if you absolutely have to throw money away, then by all means go with PG. We will continue to cruise with Ren, and essentially will be able to take two wonderful Ren cruises for the price of one on PG. You be the judge! Be sure you compare apples with apples. Flotsam
Be prepared to enjoy the awesome beauty of French Polynesia, which ever way you decide to go.