The only torsion bars that I have seen break have been the right bar on slant six A body cars. They are usually .83 or .85 size. I don't know if the right side is more prone to breakage due to the weight being greater on that side. I have never seen or even heard of a larger aftermarket bar breaking.
Back in my Camaro days, I'd see some 73-81 Z28s in junkyards with broken rear sway bars, sized .55. They weren't always broken but the only ones that I ever saw break were the smallest ones. The Trans Ams had bigger front and rear bars. I never saw a broken one on any of the Trans Ams.
I've never seen a broken coil spring for that matter. Plenty of bent and deformed ones though. I've seen and experienced broken leaf springs a few times.