Comparing those two FP's has nothing to do with what type of 3D each uses. The HX950 is one of the best
LCD's available today, only bested by the Elite PROX5's, and is their flagship (minus the new X900's.) The R550 is intended to be more of a value/entry-level option from Sony. That doesn't mean that the R550 won't perform admirably, I believe it will for the price, it's just comparing apples to oranges with those two models.
To answer your question about passive v.s. active, I would say that generally speaking, most people prefer passive over active regardless of the drop in resolution.
With passive 3D one eye sees 1920 lines of resolution and the other sees 540, which is where the "non-Full-HD" claim comes from.
With active, one eye sees 1920 and the other sees 1080 (hence the Full-HD claim.)
Passive 3D doesn't "pop" less, it merely has a lower total resolution when viewed, which to some is unacceptable. However passive 3D does offer more brightness which is a huge plus in my opinion, because active 3D is nearly always
much too dim which in turn hampers the overall image quality. So for your Avatar question, I've seen the movie in both passive and active, passive on the LM7600 from LG and the active on the PZ950 from LG, and I felt as though the 3D presentation was superior on the passive display because of the added brightness, especially given of the nature of the film.