I think that, at this point in the series, Chris is more experienced and commited to fighting bioterrorism, while Leon is more commited into serving the secret services, and occasionally dispatched during incidents involving bioterrorism.
I don't know all of their assignements, but I'd bet Chris has actually achieved more, since he's always been in anti-bioterrorism groups, while Leon serves the president, so most of his missions probably don't involved bioterrorism... (however we don't see them because they're not interesting to the RE universe).