I have been using original delta regulator that's over 15 years old with piston 1st stage px2, i think , with a turret design... It always breathed good but the 2nd stage was bulky and a little uncomfortable ( jaw fatigue )... The second stage also performed poorly in upside down position..
So I decided to upgrade to the delta 4 with fdx10 sealed diaphragm... The second stage is definitely more comfortable and significantly smaller.. Love the swivel.
The first stage is a bit of disappointment for me. It much heavier and more difficult to set up because its missing the turret. One major gripe for me is that the DVT mechanism is making annoying pinging noise when inhaling..
Over all the breathing performance is about the same as my old reg setup but a bit more comfortable under water and performs well in any position. Now only if it did not ping....