Obviously, higher quality, more heavy duty clamps are less likely to flex when being used at reasonable clamping pressures.
Durability.. it's not likely to wear out, ever, if used below it's rated capacity.
A flexing C-clamp is about worthless, as many are, that are commonly seen in stores. If a common, light duty C-clamp isn't being used on wood or other soft material, it's easy to overtighten them to a point where the C frame is sprung from flex.
For a cheap heavy duty C-clamp, buy an imported ball joint press kit. That's about as heavy a C frame as you'll find. The kit I bought was over $200 less than the referenced Wilton clamp. Better yet, but not cheap.. buy a quality brand name domestic tool maker's ball joint kit, which is likely to be a higher quality alloy, not cast iron.
If space isn't a problem, there are accessory C frame attachments available for 10-ton Porta-Power cylinders.. I've got one, and it looks like a 10-ton micrometer when assembled. It weighs considerably more than the 20k lb capacity Armstrong clamp that weighs 10lbs.