In regard to the ride: after studying the spring specs for a good long while, I don't think there will be much (if any) difference in the empty ride quality among the SRW trucks. The front springs are identical in all of the CCSB SRW diesel trucks. They all have the 5600 pound front springs. I haven't studied the other configurations, but they are probably the same also. Only if you get the heavy service package, the camper package, or the snow plow package do the front springs get stiffer, at 6000 pounds. The rear springs are rated in 3 stages, each getting stiffer than the last as the load is increased. The only significant differences are when you go from a SRW to a DRW, then the springs get a lot more stiff in all 3 stages. Or, in the last stage on the SRW trucks: the 250 with the camper package, or the 350. I think the difference is that those trucks have that extra leaf that contacts the bumper/pads on the frame when load is increased.
As far as the brake booster, I've always thought the hydroboost was better because there is no diaphragm to rot out and because they are a more heavy-duty design. Thus they are on the 350 and not the 250. I was leaning towards getting a 250 before I learned of the brake difference. Now I'm leaning more towards a 350 if a test drive and examination of the rear springs confirms that the ride is pretty much the same.