Brass tube (aka original system...aka closed bolt) and open bolt use different top seals and feed lips for the mags. Otherwise the mags are the same. Coversion kits for mags are dirt cheap. The bolt is different...otherwise (I think) the rest of the rifle is the same. Can't interchange the two mags (i.e. for sharing/etc...)
Up to you. I don't doubt that they'll all be open bolt eventually. The only reason the guys around here have the brass tube ones is that a lot of them were early adopters and that's what it was.
I haven't seen the adjustable valve for the open bolt system. The brass tube version is a Negative Pressure Air System.
WA/G&P systems are Positive Pressure systems. The WA/G&P & Co systems are MUCH more realistic in all the bits and pieces. So from a builders perspective, it's sweet. Getting good mags is a PITA and they're $90+ each. Heavy too.
Both systems obviously work...just two ways to go about the same thing. Personally I think that the NPAS system is simpler (less parts/pins/bits). TM Hicapa's and 226's are NPAS systems if you want to imagine the nozzle/floating valve assembly.
Guys swear by the KJW...haven't fairly spent a lot of time with it myself. The way that some parts return is a little goofy for me (Amos doesn't like it when someone says there's an elastic band in it
). And I'm personally not a fan of the thermold mags...they're big and I just don't like the looks of it. I think there's just one or two guys around here that shoot it.