Be aware that none of the three (air bags [aka air springs], overload springs or air shocks) will actually do anything but jack up the rear of the body and also create a better ride under load -- They do NOT level the suspension or change the load capacity -- If your front wheels are too light
without spring assistance, they will be too light
with spring assistance because no weight
is transferred to the front (like with a WDH).
Anyone with air bags who doesn't believe this should put the front axle
on a scale and then raise and lower the rear bag pressure and see how much the scale does or doesn't change.
I had air bags on my 82 D150, but they turned out to be overkill -- Just a little pressure put the body back to level and even the slightest leakdown would have it sagging (and it is bad to run them empty because the bags get damaged -- OTOH, if the vehicle is to be jacked up, the bags MUST be emptied or they may be damaged, so you have to be careful to remind service folks of this) -- The overkill might not be so bad if the bags are designed for a particular vehicle; my setup was designed to fit a number of trucks and mine was probably on the light
end of the range.
If I were going to do it over, I would get the overload springs (aka helper springs) because they cost far less, can't leak and are less likely to have any sort of problem -- Additionally, because most only come into play when under load, require no adjustment for heavy or light load.
I've had air shocks in the past and found them to be effective, but with all the air bag problem potential listed above, plus the air shocks mount to a far less robust fitting than do the air bags/overload springs -- Wnen I removed an old set of air shocks from my truck, I found the mounting stud on the axle
was bent significantly...