I can't offer any direct experience as I don't have either, but what I DO have a Backdraft 8E, which shares most parts with the Caldera, and my dad and neighbor both have Volcanos. I've been looking at 1/10 buggies and wondering the same thing as you.
The Backdraft is much heavier duty than the Volcano. In some ways this is a good thing (tougher dogbones and axle stubs, etc) but the extra weight (which is quite a bit) means it still breaks just as much anyway. The Caldera is surely a bit lighter than that but heavier than a Volcano. The Volcano seems more fragile while the Backdraft seems more overweight. Despite the fragile nature, though, the Volcanos and my Lightning can stand up to an awful lot, and my Lightning seems almost bulletproof.
If I had it to do over I would probably have ordered a Caldera instead of the Backdraft. But it would have been close.