[chaz]

I've been looking at all of those myself.
I have come to the conclusion that the Raven set, although nice, lacks some of the nicer features of the precision and palsonite sets. Bearing spring guide, hardened steel and better piston vs the raven set.
And from what I can tell, the difference between the precision and palsonite set is the durability, as palsonite is a form of metal hardening.
Not sure about the vacuum part, would be best for the pros to jump in and answer that one as I am curious myself.

I plan on going with the palsonite set (Not sure of HD or VC yet), with stock barrel polished inside, homemade paper spacers the entire length of the barrel, stock hop up unit with clear rubber and stock trigger (from what I have read they are built like tanks).