The big advantage to 6v is the standard batteries you will find are true deep cycle batteries which is better for house loads.
You can find 12v deep cycle but the ones you find at wally world really aren't.
A pair of 6v will typically have similar storage capability to a pair of 12v (similar size, weight and true deep cycle). Each cell in the 6v will be bigger since there are fewer. If you get quasi-deep cycle, the 6v are likey to have greater storage capability.
Catastrophic failure is unlikely if you maintain the system. If you don't maintain the system, you are likely to have problems with the 12v batteries also.