My front bumper is two pieces of 1/4 inch walled pipe...rear bumper is 1/4 inch square channel with 3/16 inch tire/jerry can carrier. Both are pretty much indestructable. I would go with the 1/4 inch. The difference in weight will be made up by the difference in strength. You shouldn't have to worry about much with 1/4 inch up there--so long as your welds are strong enough.
Measure once, cut twice...or is that the other way around?
I used 3/16" tubing for my front and rear bumpers (the rear has a swing-out tire carrier with hi-lift and gas can). The back has been there for about a year and I have had no trouble with it at all. From my experience, it's not worthwhile to go with the 1/4" tubing.
Now that I think about it, I was concerned that my 3/16" bumper would be too strong to actually act as a "bumper" (I still am). In other words, it's stronger than the frame, so in a low-speed collision, the bumper will probably transfer the shock directly to the frame with out absorbing any of it by deforming. However, I haven't had the misfortune to test that hypothesis.
Ive built a number of bumpers out of .120 square tubing. Pretty much as thick as your frame is and so far with all the abuse the ones Ive made have seen there have been no damage to the bumpers. Personally I think 3/16 or 1/4 is overkill unless you're using C channel. Just MHO though.
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