Hi, Dean. Steel would be the advantage in that it's a lot more forgiving & you can weld it with GMAW & come out good & not be welding on it for the next month if you were to weld with GTAW. Weight will, of course be more & rusting can occur as well, but from strictly a fabricational standpoint, steel is going to be a heck of a lot easier to weld, cut & bend without a ton of special tools. AL is great, but you would want to have enough confidence in your welding ability to assure that you don't end up having leaking & cracking problems down the road with 100 gallons of $4.35 diesel in it & on the back of your truck, 300 miles from home!!
If you were to run into a problem doing TIG on AL & had a bad fit up, or a blowout on a critical edge weld, things could get dicey real fast & AL isn't all too forgiving with problems like porosity, burn through, cracking, etc. Stainless would be the bomb, but you're wallet better be fat, or have access to good usable scrap sheet. Bending & shearing it is a nightmare as well if you don't have some heavy machinery to do it with. SS sucks for TIG though, if you don't have a LOT of patience & experience with thinner metals. Sugaring & other problems, including warpage can quickly arise if you're not careful.
I would NEVER want to discourage anyone from a learning project, I just have been there myself taking on ambitious big projects without having all the details mapped out first & then end up with a wicked mess on your hands halfway thru. If the materials are free, I would do it in steel, around 12-14 gauge min. You could easily fabricate it, make some mistakes & still have the ability to fix it without a major headache. That would be my $ .02 on it. Best of luck!! S.W.
"I feel so alone, gonna end up a big 'ol pile of them bones." (Them Bones, Alice In Chins, Dirt LP)