faber's will float more butt up which makes them kind of annoying when in doubles depending on your wing, especially when in a drysuit. I haven't gotten the new ones in the water, but they say they are the same buoyancy wise so I can't imagine they're different trim wise.
For me it would be a cost thing. I'm cheap, especially with tanks, so I would just get/keep the PST's. Granted I just traded a set of PST HP100's for Faber 85's, but that was for sidemount. I still have my PST120's for backmount doubles and have no desire to use anything else for doubles but I am quite tall so they fit me. If I was shorter, I would use the PST100's and have on many occasions and they are awesome tanks for people of "normal" height