The D400 has much better subjective breathing performance. Air flow is very smooth and due to the combination of the angled diaphragm and the coaxial diaphragm and exhaust valve, it has very little case geometry fault and the worst case for what little fault there is occurs in an unusual attitude (looking up at 45 degrees). In a normal swimming position it has no fault.
In a conventional reg (like the S600) there is a vertical difference between the center of the pressure on the diaphragm (the center of the diaphragm) and the highest edge (in the water column) of the exhaust valve. The orientation relative to the water column changes as you look up, look down, etc, but in a normal swimming position looking down at 30-45 degrees, most second stages have about a 1" difference between the center of the exhaust valve and the top of the exhaust valve. This means air leaks out the exhaust until the pressure is equalized in the case. This effectively moves the diaphragm inward until it contacts the lever. To prevent a freeflow, the spring in the poppet assembly has to be strong enough to resist the diaphragm and lever from opening the valve. So, that means the reg has to be tuned to not freeflow in the worst case position (looking straight down), which unfortunately means the reg is then detuned in a normal swim position.
That's where the D400 shines - there is no detuning at all in a normal swimming position and even in the worst case position the CGF is only about .5". Sadly, all that gets over shadowed by work of breathing numbers and the D400 does not get full credit for the very natural feel it has compared to some regs that get better numbers by having over sized exhaust valves and an overly positive pressure inhalation cycle.
So...WOB numbers will give the edge to the D400, but any one who has dove any other than the latest D400's (assuming it was properly tuned) and pretty much anything else will tell you the D400 breathes better.
The D400 declined over the years due to Scubapro detuning it to meet a really stupid EU freeflow requirement, due to a plastic orifice they inflicted upon later D400s and a two sided lever that was supposed to increase flow (and increase it did not need) but instead just increased the inhalation effort, and poppets that were a bit less supple and harder to tune. With the new poppets, it helps to run them through an ultrasound machine for a few minutes, but many techs don't know that and many don't know how to properly tune one. But if you have a pre-1994 D400 with the metal orifice and have it properly tuned, it's a superb reg.
Scubapro still supplies annual service kits and although hard parts are not produced, they are still pretty common unless you break something unusual on it. If you can avoid it, I'd stick with the D400 until you absolutely have to switch. Then I'd argue for the A700 as the D400 was THE top of the line second stage when you bought it and they should do a comparable replacement with their current top of the line second stage. Failing that, get the G250V as it's a bit better than the S600 and if you get an S600, make sure its the latest version with a metal air barrel - don't let them stick you with an older plastic air barreled version of the S600.
The Mk 20 morphed into the Mk 25 and there is not a lick of performance difference between them.
and will say that you will notice quite a difference. I do when I switch from my D300 and D400 that I use on my stages to S600 or G250V that I have on my doubles.
