I would highly doubt that the assembled option means that the cabinet is welded. I would think being welded would be considered a selling feature, and they would be advertised as such. It probably just means that you are paying for the convenience of them piecing the cabinets together for you.
Studying the images I noticed seams on the Jumbo Heavy Duty model, down low between the kick panel and side panels. Those seams wouldn't be there if the cabinet was welded. Also, on the Heavy Duty model there appears to be a row of hardware on the inside connecting the back panel to the top panel. Again, that hardware wouldn't be there if they were welded.
My employer recently purchased storage cabinets from
http://www.lockers.com. They purchased the Heavy Duty Standard model (36x24x78"). We had to assemble them. They are extremely stout. They are listed as being 300lbs. on the website. That's probably the shipping weight, but I'd bet the assembled weight is darn close to that. The Uline model of the same size is listed at 157lbs. I know it's not the size you want, but the prices are comparable for the same size cabinet ($375 at lockers.com vs. $385 at uline.). Lockers.com offers welded cabinets as well, although the selection is small.