Actual lbs to pull will depend upon the base wt with people less 26000 lbs, max GCWR of those trucks because of the allison 1000 in the 4500 and the 2100 in the 5500's.
Bumper pull you can go to 15K or so with a few recievers out there, or a pintle hitch frame mounted. 5W will be about the same, as one of those trucks will wiegh in at about 8-12K lbs depending upon the cab, 4wd vs 2wd yadda yadda.
Depending upon how much you need to pull, a GM 6500/7500 or other manufactures class 6/7 rigs would be better for much more than 15k or so, due to better tranny, axle gear, and engine combos.
MartyB
05 DW3500 D/A crewcab
2000 C2500 reg cab 350/auto
mem# 2825