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.
the lightning is tough. It shares most of the same parts with the tornado and volcano, but due to the smaller tires (rotating mass) and shorter suspension travel it takes less abuse when it crashes or flips.
On the volcano, most likely parts to break are rear hubs, "C" hubs, dogbones. Dogbone breakage can be limited if you shim them with nitro fuel tubing in the drive cups. Hub breakage can be limited by using the camber link holes closest to the base of the hub.
When you do the above to the volcano, its a fairly durable rig. And if you also buy the program card for the ESC and turn down the punch control and lower the initial braking force then the drivetrain will also hold up better (same goes for the caldera, as it has the same ESC as the volcano)
The backdraft is a beast. I regret selling mine. Never liked models that require 2 batteries to run, but that thing was fast and handled very well.