How do I know if a reg has the "newer jet piston"? Also, I don't see many *2 regs on ebay, but there are more *1's. Is there much difference between say a B1 & B2?
The first stages are exactly the same in the B1, B2, Z1, and Z2. (Except for the swivel turrets on the swivel-less Z*'s, of course. But they are actually interchangeable parts.)
The B1 and Z1's started out with non-Jet pistons but had them come standard mid-run. They can be installed in the older regs, and I think the upgrade kit was ~$80.
You can tell the non-Jet piston models by the rubber trim part at the HP seat cap end. The non-Jet ones have donut shaped rubber trim that exposes the HP cap (at the opposite end from the turret) whereas rubber trim piece on the ones with Jet pistons mostly cover the HP cap.
The Jet piston is said to solve an IP creep issue that happens occasionally before service is due when used a lot with HP tanks, but many people had no problems with the old piston. (I think my first B1 had the issues before I got it upgraded.)
It also is said to increase flow, but I doubt it really makes any real world difference.
The B1 and B2 second stages do have some differences, like the wider exhaust tee, (which is also interchangeable, if you like the B2 tee better, you can put it on a B1, and I think it was several dollars or some such) and I hear the newest B2 and Z2's have a slightly different case geometry that improves breathing effort. I'm sure you can also get this new casing for not too much, (I'm guessing the titanium air barrel, lever, spring, etc. are the expensive parts) but I don't really feel the need to upgrade, and all my Atomics have the older second stage body.
In summary, if you want the B2, then you will most probably be quite happy with a B1. It's 95% the exact same regulator. If you are buying used, IMO the only reason to get a B2 at the current premium is for the second stage swivel IF you want it, as it might be cheaper to get it together. But then again you might be able to get the swivel for real cheap too, as I see a lot of people going long hose trying to get rid of their now superfluous swivel hoses.