Eisenhower: military general officer, university President
Kennedy: Congressman, Senator
Johnson: Congressman, Senator, VP
Nixon: Congressman, Senator, VP
Ford: Congressman, VP
Carter: Governor
Reagan: Governor
Bush I: Congressman, Ambassador, CIA Chief, VP
Clinton: Governor
Bush II: Governor
So, kind of a mixed bag as to which makes the better (successful) President. But, on balance, I agree with your point about governors having more executive experience than Senators.
Not to be picky, but Nixon was never a Senator.