My router (about 1500x750 cutting area) is all steel, welded. I have only used bolts where I fix to aluminum mounting plates for motors, etc. I was also a first-time welder on this project (using MIG) and an angle grinder fixes many problems

Aluminium profile is bolted because you cannot weld it (at last, not many of us can, or have the kit to do so). It's probably a good choice for smaller machines but as the size goes up, the balance shifts towards steel. Cost becomes a strong argument then. I guess that if you are going to need to break it down for transport, say, bolting might be an option but you are going to end up fabricating brackets, welding on flanges for bolting, etc, so welding the lot is less work. All these things are a balance, a compromise, but it's probably easier to cut and reweld something that went in the wrong place than to move a set of bolt holes!

I can only emphasise a couple of points Jazz has made - if something might need adjusting, make sure that you allow plenty for it. Even the steel box you buy is not straight or flat and by the time you have welded it, it will be worse! That's the place for bolted, movable, joints - for setting-up tweaking, not the basic structure. And I can tell you from my own experience, 50x50x3 really does resonate - there's no mass damping worth talking about. I'm doing some cutting in steel at the moment with my machine, making some fiddly little bits that would be a pain on my manual mill. I never expected to be able to do that when I built it - it was intended for wood - but it can just about handle it. I do wish I had gone up a size or two in wall thickness, though, just for better resonance control. Search for "AVOR" for my write-up to see more of my mistakes...

Plan carefully - it's too easy, especially on the Z-axis, to paint yourself into a corner in the sense that you can't get to half the bolts to assemble it. 3D CAD, if you can use it (and it's worth learning), can be a lifesaver. For example, my Z platform has a couple of critical bolts that hold the ballnut in place and need tightening only when everything is assembled and adjusted but with good planning I made sure that I had already added a couple of access holes for the Allen key in advance.

Have fun - it's a frustrating game and there'll be lots of sometimes conflicting advice but it's great when you start cutting on your own machine!