I know I won't be going through the rocks like some of you guys do, so I figured aluminum would suffice...and it looks good too!
The main reason I didn't go with the rivnuts was because many of the old holes became misshapen after the removal of the screws and old nuts, and I have read that rivnuts can be finicky to install at the best of times and if the hole isn't close to perfect, forget about it. I had to replace 18 of the 24. While in a great local fastener shop ...one where they have EVERYTHING on the wall...scoping out my options, I told they guy working there what I was intending to do, and he went and grabbed a co-worker who has experience 4x4ing. This guy directed me to 1/4-20 rubber well-nuts. "Rubber?" I asked, and he said "Yup!" He then took me out back to the parking lot to show me his "jeep." This thing was a rock crawlers dream! He proceeded to show me the lizard skin and large flared fenders he had installed...all with well-nuts. He had tried to use rivnuts and gave up on them and went with well-nuts. I asked again about their strength and he told me his have stood up to some serious scrapes, not to mention he can stand on his fenders...he wasn't my size, but about 190lbs. He also likes the fact that they can be removed and re-used.
Anyway, they aren't cheap, and I did have to widen the 10MM holes to 12.7MM (1/2") to fit the well nuts, but they went on great! Now, can they hold my weight if I stood on them? Probably not, but I found a website that tests these types of fasteners, and the tensile strength was listed at 280lbs! What I also like about them is that they completely seal up the holes....being rubber and all...and they leave a nice gap between the truck and the rail, allowing water to pass through. I will get some pics up shortly.