As a side note, people say you hit your rear bumper more than anything. Is this true? I expect to bump the engine plate now and then when I misjudge an obstacle. I am not gearing up to do rock crawling, that is something else entirely.
The full set of RCI steel plates for the 4runner (including the huge fuel tank plate) weighs 174 pounds. This is the weight of one passenger with an athletic physique. Aluminum plates might save 50 pounds. That is not a big deal.
They make these points:Steel is heavier. Steel is also stronger. If you plan to make a career out of scraping your way across rocks, the steel plate is the way to go. I view a skid plate as insurance. I have done a lot of 4x4 driving with my Tacoma (with no real skid plate) and just avoid situations where smacking things is likely. In fact I view smacking things as "not an option".
So if you intend to drive just "nice dirt roads" and expect to never touch your plates, you might opt for aluminum and have something there just for insurance.
Tom's Auto repair pages / tom@mmto.org