I've used both Bilsteins and Ranchos. Here are my observations...
Put Bilsteins on my '96 RAM 3500 CC 4x2 because of their reputation. Enormous improvement.
About a year later, I switched to Rancho RS9000s because of a special group price we got from 4WheelParts. I found that, at least in the rear, the Ranchos did a better job of controlling camper sway, since I carry a large Lance camper. The fronts, however, were fairly stiff.
When I traded and got my '01, the rears moved to the new truck, and I put new RS9000s up front. I found that 3 was good unloaded and 4 was for when the camper was on the truck. For the rear, I ran 2 unloaded and 5 loaded. These were the five-position shocks.
Then I got my '03 a month ago. I had recently put new RS9000X shocks on the rear of the '01, and I took them off and put the old worn out RS9000s on before I traded it. The RS9000X shocks fit the new truck just fine.
Then I decided to try something different. I put Bilsteins up front this time. They did cost a bit more, but I wanted to give 'em a shot. I had been complaining for a long time that my '01 rode very harshly and I suspected it was a combination of the Ranchos and my 19. 5" wheels and tires.
Well, lo and behold, the truck now rides really nicely. The fronts could be a little stiffer for camper hauling, but they're very liveable, and the ride quality improvement, especially offroad, makes it well worth it.
I will continue to run the RS9000X in back since I can tighten 'em up (I run them on 9 with the camper, 1 without) as needed.
I suspect that Bilsteins in the rear would be ideal if you aren't hauling something top-heavy like a camper. I would definitely recommend them.
Rob