Steel is cheaper and most of us hobby types aren't set up to weld aluminum. (I'm not anyway). Aluminum is alot easier to melt or burn and poor welds are far more likely to crack (Catastrophically). If you're going to weld it yourself, and you don't have alot of experience, I'd suggest using mild steel as it's much more forgiving (and far cheaper).
If you are good friends with someone who does some welding maybe you can work something out between you. An actual welder is likely going to want at least $100/hr to work on your project (Rates can go much higher, especially if you're looking at something like TIG welded aluminum), so if you cut all the pieces in advance, you can save alot of money. Alternatively, if you have a MIG welder, but don't trust yourself to do the actual welding, you can tack weld the whole thing together and have a shop run the actual beads. This will take far less time for them and will save you boatloads of money. Grinding the welds and finishing the stand yourself will be a big $$ saver also. A welded stand will be much sturdier and less wobbly than a stand put together with fasteners, although a well designed stand would be plenty strong enough, even if bolted together.
We use those bars Koop linked to for temporary assemblies at work all the time, and they're pretty sturdy, although I've never tried to hold up half a ton of water with them.