according to our Yakima and Thule Rep they recommend no more then 150-200lbs but they will hold much more then this. the support is the biggest concern the load rating is going to come from your cap manufacturer not the rack itself.
as far as which rack if you want to be able to take it on and off easily go for the Yakima landing pad and control tower set up. its super simple and you turn the 4 bolts in the towers and it unlocks (similar to a door deadbolt) and the whole rack will pop off. the Thule system will give you more bar spread adjust ability with the tracks and works excellent as well
Support Search and Rescue...Get Lost
2003 Ranger Fx4 4.0 TR- DBR long travel, Deaver G50, 305/70R16 MTRs, Supercharged, C4x4 bumpers, super sliders, 10K winch lots of lights and radios.