well dom stands for drawn over mandrel meaning it is seamless tubing.  hrew is rolled over and welded at a seam.  so dom is stronger being one solid piece.  you also have to think that the smaller the tubing, the less it will dent because there is less area inside the tubing for it to push in, and a smaller diameter means the tube is a tigher overall circumference which makes it stronger.  also wall thickness comes into play.  i use 1 5/8" dom, because it's unique and no one uses it.  but i'd say use 1.5" dom with a .120 wall for toyotas, and heavier full size rigs (dodge chev ford) i'd use 1.75" dom with a .156 or .188 wall tubing.

Not exactly correct in your assessment of DOM tubing, as it is a popular misconception that DOM is not welded, when in fact, it IS is welded.  From http://www.steeltubeinstitute.org/dom.htm as it is explained there well.

"The manufacturing process for DOM tubing begins with coils of steel, which are slit to the proper width for the desired tube size. The strip is cold formed and passed through an electric resistance welder which joins the edges together, under pressure, to complete the tubular shape. After testing the weld's integrity, the tubing is cut to length for further processing.

The cold-drawing process creates a uniform, precision product with substantially improved tolerances, surface finish and tensile strength, increased hardness and good machinability. In this process, the tube is cleaned and annealed, and one end of each length is squeezed to a point so it can be gripped by the drawing mechanism. The tube is then drawn through one or more dies and over mandrels. This reduces the diameter of the tube and thins its walls to the required dimensions in a controlled fashion to provide the qualities desired in the finished product. Metallurgically, drawing improves the tube's concentricity, tensile strength, hardness and machinability. Close dimensional accuracy is achieved through tight control of both outside and inside diameters."

And I'll let you read the various stuff about ERW for yourself... http://www.steeltubeinstitute.org/hss.htm

I'm going to use 1.75" DOM 0.120" wall for my exo.  Properly triangulated and braced, it'll be rock solid.