C12 is simpler design and slightly smaller with only a single turbo.

(no second turbo, no precooler, no VVAs[variable valve actuators], no bellows inbetween 2 turbos and/or precooler to crack and leak boost, no VVA solenoids to fail, no water divertor valve, no VVA oil pressure sensor to fail, less overall wiring) This is just for the pre-2007 C13 engines.

You can triple those differences on the later 2007 and newer C13 engines.