These fail together, I'd call them equally safe. Using an unusual (and small) stack is safer as exploits would be more expensive to obtain.
Probably better to airgap by having a secure microkernel (L4, how are you?) do the USB and another protocol (Ethernet for all I care) carry sanitized payload to the actual machine. Think of it as wearing a condom. Whatever the transferred payload is, making sure it's sanitized is vital and non-trivial. Probably would require interpreting and serializing it again, to unify the formatting.
