I have the B2, and LOVE it. It's so easy to breath with the B2 that the first time I used it I had to adjust my inhalation effort to keep from sucking up a tank of air in record time. The swivel on the second stage makes the ultra-light second stage even more comfortable, and the mouthpiece is also comfortable. The control knob is nice to have on the surface, and is extremely easy to operate, even with gloves on. Purge is easy and efficient.
I had considered the Z2, because it has the same technology as their other regs, but if I had added the swivel 1st stage, and the swivel for the second stage, the price was too close to bother.
Have read some good reviews on the Oceanic Delta4, but have also heard [all hearsay, I don't know first-hand] that Oceanic has some internal challenges. Might not effect the reg itself, but could make it hard to get parts.