I've heard both of those are good, and both do best if the prep work before painting is done according to the directions. Neither one will help with the internal rust inside the "frame" but they would probably extend the life of the Jeep's undercarriage.
My neighbor swears by Chassis Saver and he has a 1993 GMC pickup with a 4 inch body lift and BFG's... The Beast, he calls it. I think the Chassis Saver has worked well on his rig. His chrome bumpers are rusting through from the inside, but the frame and the majority of the truck body are rock solid.
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2008 Liberty Sport 4x4 Automatic
Jeepin By Al 4" Lift Kit
BFG KO2’s 245/75R16
Full MOPAR skid plates
Rock Lizard Skink Super Sliders
AtTheHelm Front Recovery System & Roof RackM116A3 Trailer build thread: viewtopic.php?f=72&t=77997