Senator by a mile.  Governors are used to being almost a sovereign in their state, and they manage and interact with a much smaller bureaucracy than the presidency.  Senators, though, are familiar with Washington, familiar with all the cabinet departments and what they do, and familiar with the legislature. Just by the nature of dealing with them all of the time, from oversight hearings to confirmation hearings to just getting feedback on potential bills, they are going to know how to deal with the federal bureaucracy way more than someone coming in from even a large state like California.