Hi bimmerZ5
Good question. Nor is it an easy one to give short answers too. Quality is defined by the customer after all. Customer requirements also change over time...
I guess I look mostly at form, fit , function - followed by structural design, materials and construction techniques.
Not consciously at the time, but now looking back at what I usually - actually do; I look at form - the layout in terms of what tools I have or will have - and where would they go in a tool box. Usually I find myself pulling out the drawers - at first about an inch - then I try to wiggle them side-to-side. I also then pull the drawer to full extension - and apply opposing pressure to each side of the drawer - up on one side - down on the other. Just to get a feel for the rigidity of the drawer and an idea of how they would function when loaded over time.
If the tool box passes the initial look and feel test - usually I look more closely at the structural design, materials and construction techniques.
The sheetmetal in both my SO and the HF boxes would seem to be about the same - relatively thin. Both simply bend sheetmetal to form the bottom and sides of the drawers. In my SO box they use drawer dividers inside the drawer s, either front to rear, or side to side - to reinforce the drawer. Kind of an "T" beam suspended between the two sides or ends and adding strength to the bottom. The HF box uses a boxed sheetmetal frame under the drawer for the same reason - but it leaves the drawer fully open inside. {a nice feature IMHO}.
The drawer slides on my SO box are spot welded to the sides of the drawers. This adds greatly to the strength of the drawer - and all but eliminates side to side movement. The down side is that you have to use a "tool" to remove the drawer and the slides are not roller types - so effort to open/close them when fully loaded is pretty high. {Newer SO boxes may be using roller or ball bearing slides now}.
The drawer slides on the HF box are roller type and they are not welded to the drawers. HF uses rivets to attach the slides to the sides of the drawers. So you do have a bit more side to side play - the upside is that the drawer slides could be easily replaced if necessary - and they can be removed with the flip of a latch. The slides themselves on both boxes are certainly strong enough to hold a drawer full of tools for many many years. On the HF box, with drawers more than 4" deep - they use two slides on each side of the drawers as well.
On both SO and HF where sheetmetal is overlapped - like the front or rear ends of the drawers - they are welded together. HF folds the sheetmetal over on the top edge of the sides of their drawers - about 1/8". This adds strength to the sides. SO doesn't - I believe because welding the drawer sides to the sides of the drawers accomplishes enough strength.
I'll mention that both SO and HF top boxes have their lids attached via piano hinges that are riveted to the back & top.
Both are painted - my SO is bright and shinny - making it relatively easy to clean, the HF box is painted with a rough finish - I don't know how that will stand up over time, or how hard/easy it would be to keep clean.
The HF box has nicer casters - they are larger in both diameter and width - than my SO box. But then the HF box is more a "roll cab" which is quite a bit wider than my SO which is more a "bottom box".
I worked in an engineering environment for 30+ years - and we could always debate any technical detail. Everything seems to be a trade-off of some type. No question that if we got a group of guys together - sat the SO box beside the HF box - we could argue endlessly about the overlap areas of sheetmetal and how many spot welds or rivets should be used. Likewise we could argue about how strong something really has to be to do the job and how much margin should be included in the design etc etc.
All that agreed too - I think that the HF box stacks up pretty well in terms of design, quality build and materials. Personally I like some of the design features of the HF box a little better. I am also quick to admit that I bought the SO box mainly because I wanted to give myself the top rung item at the time - - one that 99% of the mechanics and car nuts I know agree is the "BEST". I could easily afford it and I've always been very happy with that decision. Now twenty plus years older and wiser... I don't know if I'd make the same choice today. $400.00 for the HF box vs $4,000.00 for the SO.... Or for that matter $400.00 for the HF box vs $1500.00 for a used SO...??? All depends on what you want far more than any difference in objective quality trade-off's.
I'd also say that both companies are giving their customers what the Customers want.
Like I said - I'm not saying that the HF box is "as good" - just that in most regards it comes pretty close for one tenth the money - and that it is perhaps as good as any home garage will ever need - its a pretty "smart buy".
FWIW,
Carl B