There is more to it than that. Where do they attach? Torque box or rear frame rail? short or long? How are they attached, 2 or 4 points or seam welded. And of course how do they look (and how low do they hang).
The tubular ones are the Global West pieces. I'm not exactly sure where they attach but I am sure that you can only weld them at their front and rear attachment plates.
The square ones are from Chassis Engineering. They are pretty much sight unseen since their website doesn't show them. Worst case, they attach front and rear like the GW ones. Better case, I can weld along the length.
I was trying to figure out which shape/thickness would support the body better. I have a stress crack in my paint at the top of the A-Pillar and one on the rear valence (strange) so I need to do something. I'm thinking the round ones with thicker wall will be better but I'm not 100% sure.
The CE ones are cheaper as they come unpainted. No big deal to paint them though...
If you are looking at the GW sfcs, I'd look at the Maier Racing ones as well. They are longer and and attach at 4 points, which distributes stress over a larger area. Maier didn't make them for convertibles when I put GW on my car. I'll probably be taking the GW units off soon (or welding more braces to them)
I have no experience with the CE units. Good luck.
I have the Maier racing ones on my 69 coupe and when I put a jack under the front left tire, i can get the rear left tire off the ground, and the right side tires were left on the outside edge of their tread. That is pretty impressive.
The key to the Maier subframes are that they dont just fill in the space between the front and rear frames, they extend approx 2 feet beyond. this eliminates a pivot point at the single connection that most subframes have.
if you make your own, I would use tubular tubing and run it longer beyond the framerails like Maier