We talked about that here. Again it depends on the composition of the aluminum...
FYI I checked out S4XC's tcase skid. Its 3/16" 5052, but they don't state the temper condition. I'd guess its 1/4 hard since that's the most popular..... which puts it at about half the strength of general purpose steel. BUT its extremely corrosion resistant.
I've never scraped aluminum across rocks, but the way I see it a softer metal will allow jagged edges to dig in and grip better. But hell, if people are using them for competition I'd like to know what grade their using and how they hold up to impacts..... nothing beats trailside experience!