Racks are one case where you can get away with SS. The reason is that racks are often silver brazed, and with your fuel choice that is pretty much your only serious option. You have to silver braze SS (or weld it), so you are already going down that path.

The other thing is that while SS is crap compared to 4130, unless of aero boutique quality, it seems to work for racks. Some big name guys use it, and it holds up. They are basically using brake line stuff, or the Aircraft Spruce stuff (I would personally choose the AS stuff). You essentially don't have a choice as to the wall thickness you use since it isn't really being sold within the structural size grid one gets with 4130 (no stainless for longerons, and for ribs, and for load pieces as with 4130). I think your main choice is 3/8", 1/16".

WIth 4130 you have a lot of choices, and you can't really specify a wall thickness. First, .028 tends to be for the light racks, however 5/16 and 3/8 tube are very different structurally, so the tube diameter has to be considered. You also have to separate the pieces that are being used perp to gravity, or radial to gravity. Just like a house frame, 2x4s for posts, and 2x12s (or all sorts of different dimentions for beams, depending on span). Another thing are parts being used as struts, they are loaded moreso in-line which is nice. Tension is normally kinder than compresion in in-line loading. Also, how are you taking the loads off? Are you turning elegant "drops" or mashing the tubing flat? And the design is a big factor, is it minimal looking or are there lots of load sharing parts.

I'm not suggesting you need to be a structural engineer to do this, but look at your design. If it carries more weight with less parts, and the horizontals are cantilevered a lot, use heavier walls, and heavier diameter. If it has a zillion little trusses, lighten the parts up. Put the weight where it maters, take it out where it doesn't.

With silver and any of the tubing up to 3/8" you can get by with mapp gas, but it won't be as easy as it would be with hotter torches. However, a cheap OA rig may not have the control you would want for doing this work and will be too expensive on larger projects if it uses bottles, so I would just give it a pass.