There are two factors to consider here:-
The pressure: This is the pressure of the nitrogen surrounding the bladder of the sphere. The higher the gas pressure, the less the LHM filled bladder is able to intrude into the gas half of the sphere, therefore stiffening the ride as the bladder is not able to stretch as much as a low-pressure unit.
A heavier car appears to always have a higher pressure rating and a lighter one lower, this is with the exception of more "luxury" Citroens that inherently require softer setups i.e. high spec XM's. For this reason our light BX's would require the spheres from a heavier car to stiffen the suspension.
This is the opposite to normally sprung cars where a "heavier" spring would cause the car to ride higher due to not correctly compressing the first phase of the dual rate springs (and i believe this is where most peoples confusion over the subject arrises).
Of course, with our self-levelling that isn't an issue!
The other consideration is the bore of the hole which allows the LHM into (and out of) the bladder. The smaller the hole, the slower the fluid moves and thus reduces the speed at which the damper moves up and down (equivalent to "firmer" uprated shocks on a normal car). 0.5mm is the smallest available upto around 2mm.
Rating are given as 500/55/1.1 meaning:
Sphere Capacity (not particularly relevant) /
PRESSURE (in bar) /
BORE (in mm)
You are right that there are many schools of thought, none with any real evidence, just theory.
A few years ago I was told the best spheres for sporty use were those from the rear of a 3ltr V6 XM. At 30bar/0.5mm they are the lowest pressure/ highest damped (see above) spheres available. Suffice to say after the expense of fitting them all round the handling was completely screwed and I was convinced I must have been given the wrong spheres. After learning more about them over the years I now understand what it is that's needed.
You want high pressure for stiff springing, small bore hole for maximum damping and lower the car with the height correctors. Your combination of those is your choice but when choosing, always consider the weight of the donor car and it's "comfort" level. You can always re-gas to the pressure you want.
Dowload this, it will help you understand the choices:
http://cid-6ce6986c07c4d5d4.office.live ... 0Table.pdf
Oh, and NEVER trust a motor factors word that the spheres he supplies you are for the car you asked for unless the pressures are actually printed on the sphere (this is because most factors think all spheres are the same and will take whatever the suppliers have in stock).
As I have brand new set of XM rears in the garage I was planning to get them re-gassed to 75bar and see what happens... mind you, i've been planning that for 2 years and it still hasn't happened...
Good luck and keep us posted with what you decide to go for.
JayW.