Hi mate,
I have little to no experience with racks however as a commercial builder I work with both materials plenty. A lot of people would be surprised by the strength properties of aluminium, especially with alloy composites.
Boxed sections of ali are pretty damned resilient and it takes a fair amount of force to do any damage. Generally, because of the savings in weight their thickness is beefed up more than steel as well.
Another bonus with ali is that weather elements have no real effect on it, it wont rust but may oxidise over time if in a raw state.
The only downers on ali is that thinner parts may be subject to metal fatigue quicker than steel from continually movement or flexing, and if you want to modify or fix anything you need to tig weld it - working with steel is much easier for a home job as you can mig or arc weld it.
Again, I have no experience with the racks, but as a material aluminium is pretty bloody good and there shouldn't be a compromise in strength.
I'd advise doing a bit of homework on the racks and see how thick the members and anchors are in comparison to steel. All that information should be able to be sourced from the supplier or manufacturer no problems....hope that helps a little
