I run a 35" spare on my Kaymar swing-away. Both Kaymar and Slee are excellent bumpers, but have different purposes. If you're doing mainly rock-crawling and other serious types of four-wheeling, get the Slee. Or if you don't mind cutting, get a 4+Plus with a bobbed rear frame for ultimate departure angle. If you enjoy more expeditionary off-roading, with some hardcore thrown in, get the Kaymar. It is much more versatile for expeditions (I've got a tire carrier plus jerry can carrier plus work lamp which fits into a nicely integrated mount on the back of the tire carrier, and you can get antenna holders, Hi-Lift holders, and other goodies to increase its useability).
That said, I've taken my Kaymar through the Rubicon twice with no problems (though what Rod mentions about the side wings is true [and I have also seen the same problem on Slee bumpers, as the frame and body flex independantly, and the bumper is attached to the frame], just have to ease it a bit over the big boulders...)
Weight differences between the two will be fairly negligible.