I've got Pedders foam shocks in the back of the Pathy. Unladen, they're certainly firm. Loaded up, they work well. I'm led to believe that the same shock set up as just a gas shock is a lot more compliant.
So why did I go the foam? Better corrugation/fade resistant with heavy loads, or so I was told. Up here, a lot of the dirt roads are reasonably heavily corrugated, and with the back full and the camper on I wanted a shock that could cope.
So that would be my deliniation between the two - if your 4WD is mainly a weekend playtoy in the High Country, gas shocks are probably what you want. If you're likely to be hitting the GRR or the PDR, then I would suggest the foam would be the better option.
+1 to Dazzler's comments too.
Good luck!
Matto