Major influence on breathing effort is from 2nd stage membrane diameter.
New models has trend to minimize 2nd stage size.
There is the reasons - lower water stream resistanse; lover 2nd stage bouyancy, due to air ammount inside; lower amount of inner space of 2nd stage allow you to decrease dead volume, that allow you decrease CO2 retention.
All this trends dragging down decreasing membrane size, so - membrane surface is less -> total force from membrane to the valve lever is less
(We keep by default, that breathing effort is equal to "vacuum" pressure inside 2nd stage, proceeded by lungs.)
Force is less -> we need to develop new valve, or have brilliant tiny adjustment of the valve, otherwise effot will too big or you will have freeflow.
Old regulators (2nd) had bigger size of membranes.
Once I had excellent 2nd stage, very old, and very simple -
(sorry, only this pictures I have)
It had very simple valve, unbalanced, but with manual adjustment.
And this 2nd stage had best breathing effort, I ever tested.
(late I got Scubapro BA-109, it had near the same, also with big membrane).
Also exists old Soviet AVM-5 2nd stages with very big membrane:
It even has upstream valve!
but also it have the fine breathing effort.
So, 2nd stage has difference. And new models does not mean better models.
If you WILL decide - have a look at Scubapro BA-109 (or more modern near-the-same analog G250). Most of divers, who knows this 2nd stages will confirm my recommendation.
Most or all the breathing differences will be in the second stage. If the first stage is delivering 130-140 psi even with draw down or being unbalanced it won’t matter much if the second stage is a high quality balanced model and can overcome erratic IP’s.
No, it is not true. 1st stage has very big difference.
First - is 1st stage balanced or not.
If you have unbalanced 2nd - you easy can feel difference.
When you will finishing your tank (air in your tank) - you will have sensitive difference.
BUT! Sometimes it is good! Because when you start to feel difficulties with breathing - it is the reason to make a look at SPG
With much experience you be able to feel, when you need to go back, and
"Wellcome to our sh!tty world back"
Another difference - is 1st stage HP seat diameter (and inner air channels size).
E.g. once Scubapro makes experiment (in real it was marketing sh!t, but it was in real) - asked to sit down 135 divers in the pool, and breathing from just one 1st stage
(
proof)
As example - there is old Soviet Ukraine-2 1st stage
It is nice, small sized, simple.. but it had small seat orifice, and on the depth, bigger than 20m will not get you as much air, as you demand...
So - conclusion:
- there is the difference in regulators (1st stages) and demand valves (2nd stages).
But not always new is better.
But in my opinion - new balanced 1st stages and some old 2nd stages is good!
