In my part of the world (a far flung corner of Australia) good quality steel is virtually unheard of. There is an abundance of that gummy hot rolled stuff. Local industry use it, and there are plenty of off-cuts if desired, but they want a pretty penny for scraps. And it's just awful to work with.
Ask for cold rolled steel, and you can expect a good natured shrug. Ask for anything more exotic like 12L14, or the wonderful (UK) EN8M, and there will definitely be furrowed brows. It just doesn't get used by our local manufacturers, so they never see it. Gauge plate is unobtainium, although drill rod is still available from Blackwoods.
Basically we just don't have a steel industry in our country that even remotely equates to the UK or US. If we need the good stuff, we model engineer types have to really pay through the nose, by ordering from specialist suppliers, mostly overseas/online.
Aluminum alloy on the other hand, is as common in my part of the world as water. All dimensions, grades, you name it. And I have a couple of local fabricators nearby that use a lot of it, so off-cuts are thick on the ground. (off topic - is there any better way to spend an afternoon, than by browsing the local off-cut rack, scoring some really choice cuts of aluminum for the next project? I think not!)
So when it comes to a substantial tool, my first thought is: "Can I get away with aluminum or maybe even brass?". Because I know steel is going to really hurt the wallet.
Usually its a question of wear and rigidity. For extra rigidity, I usually increase any critical dimension to account for the increased flex. For example, I think it was borderline making my Vise Sub Table from aluminum. But I stiffened it up by adding a bracing section underneath, and it sits like a rock in the vise. I don't have a need to push it too hard, and thus far it has served really well.
If the worst happens, and it becomes clear that the aluminum is not up to the task long term, well at least I have a finished prototype that has been in the field for a while, to use as the living blue-print for re-making it in steel.
The other bonus from Aluminium of course is that its an absolute pleasure to machine; No excuses for a poor surface finish!
Thanks for stopping by,