Just for anyone else reading along: The OP and I used a "toolbox density" calculation as a means of comparing different tool boxes. We took the weight of the box and divided that by the published cubic inch volume of the box. d=m/v. This can provide a rough sense for how stout a tool box is and helps cut thru some of the hype surrounding sheet metal gage thicknesses.
But there are also several problems with this approach. Boxes with a greater number of shallow drawers like the Kennedy 315XB, will appear "denser" i.e. better constructed, when in fact they are not. It also fails to consider the actual construction of the box. Corner gussets, caster support, and the number and spacing of spot welds, all greatly effect the service life and quality of a tool box.
"Toolbox density" can be used to differentiate between consumer grade and pro-grade boxes. But generally, so can price. In my study of 40" roll cabs, the sub $1000 boxes all had the lowest density numbers (HF, Craftsman, Montezuma). Unsurprisingly, these boxes also had the lower grade drawer slides. The $2000+ boxes had the highest densities, and greatest drawer capacities, best casters, etc.
In my study, only 2 boxes stood out: The HF44" box was at or near the bottom in every measure. Importantly for many here, it was half the price of the next lowest ranked box, the Craftsman Premium. For folks who don't need much in a box, this was by far the best value. The Lista HS series 94 offered nearly double the drawer load capacity of the comparable Snap On models and was 1.5X heavier.
In this particular instance, Proto has 2 different published numbers for its volume. It is also unclear whether the published weight is the shipping weight or net weight of the product. Using a bit of common sense, it appears these top boxes are all roughly comparable in terms of material thickness. I don't think anyone would be disappointed by low quality material thicknesses in any of these.
The decision between the boxes (neglecting price) then comes down to the quality of the struts, drawer locks and slides, and the depth of the drawers. Snap On typically sizes drawers to allow for the storage of specific tools, 1-1/2" combination wrenches on edge, 1/2" deep sockets in Hansen racks. It would be a good idea to ensure the drawer sizes will be sufficient for your needs.