The 'levelled suspension' springs are pretty soft, and give a nice ride. The drawback is that when loaded up the rear will sag as above - the load leveller is supposed to counteract this and 'pump up' the back end to its proper ride height. Many however have failed and I've never heard of anyone rebuilding one successfully. Not sure if they're still available new.
The 'non-levelled suspension' springs are harder, so don't sag as much when loaded, but give a much harsher ride when not loaded.
The best option for general use is the functioning self-levelling setup, nice ride but able to be loaded. My plan is to replace the function of the load leveller with a set of air bags which I can pump up when loaded to increase the effective spring rate on the rear and avoid sagging. Your best option may vary, if you carry 300kgs of kit all the time then the non-levelled springs may do for you.
There is no simple 'cake and eat it' solution - the progressive springs as fitted to later 110 SWs (when they stopped with the leveller) are good, but it's still not as good a ride as the original levelled setup.