I think, with aluminum, alloy selection is more important than with steel. When selecting aluminum for construction, you need to make sure it has the correct strengths, resistance to thermal stressing, etc. Shape of the aluminum extrusion, as well as wall thickness, will also bring strength to the construction.
I worked at a sign shop for several years (in FL) where all the cabinets were made from aluminum. The support poles were steel, mostly due to them being cheaper, and heavier (high weight was important there).
I would think that as long as you do pick the correct aluminum alloy, shape, and wall thickness, you can make a stand that will last for generations.
A good point (already made) is the ability to easily (or more easily) drill aluminum (compared with stainless steel especially). If you want, you could even make a stand using square tubing (or box tubing) with blocks inside the ends, or at connection points, that you can use as attach points. That way, you can simply unbolt it in large sections, and move/store it much easier.
I think it should come down to material availability, how much you're willing to spend on the stand, and what you're more comfortable working with. Personally, I can gas weld steel (oxy/acetylene) but I'd be willing to work with aluminum... By the time I'm ready for a stand, I should have a house, or at least a garage to brew in. Chances are, I'll pick up an electric welder, so I could have the ability to weld aluminum too. Although, there's something to be said for having a professional do it for you. Invite the welder over for a cold one (or two) and you might even get a decent discount.
BTW, cook1969, very nice use of 'recycled' materials... I've seen server racks listed on craigslist for a decent range of prices. I hadn't thought of using those, until now... Even if it's a bit longer than you plan (racks can be up to 7 or 8 feet tall), you could just use the extra space for storage/rest areas...