A carbon guard, like a P3, would spread any force over a greater surface area than an aluminium guard, like a 1-2mm bend into place aluminium guard.
That's just as much down to the design and shape of the guard as the materials used.
You mentioned a carbon guard damaging other things?, do you mean the frame?
Perhaps, but I would imagine the pipe would fail at the mounting points before it damaged the frame. You could damage the exhaust spigot if you loaded a carbon guard up in the correct orientation. Due to pipes looping back on themselves, I imagine the pipe would bend there instead of causing damage to the cylinder.
If you hit something, the force has to go somewhere, a point load on a aluminium guard would most likely result in the guard deforming and transferring load onto the pipe, which then might deform at the point load.
If you do the same with a carbon guard, the guard is less likely to deform than aluminium, and would transfer load over a greater area on the pipe. If the load is large enough, then you can still deform the pipe without visibly damaging the guard.
Generally, I would expect carbon guards to be more prone to pipe bending, and aluminium guards to be more prone to denting. This is generally for sharp direct point loading, and based on my experiences, and your results may vary.
