True, round tubing is better for roll cages and tube chassis. However, for a weld-on subframe connector, you're welding it to a flat floor plan. Round against flat doesn't match up squarely, so the round tube subframes have flanges welded on the end.
Even with the flanges, round tube subframes don't provide as much surface area to be welded as square tube subframes. The extra strength of the round tube is more than offset by the decreased surface area of the welds. No matter how strong the tube is, that won't matter if the welds crack.
That was my long-winded way of saying buy square tube subframes.
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Moxie Racing
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