I would prefer history. It provides perspective. It humbles and it requires a mind that can comprehend large amounts of information. Although I don’t always agree with Buchanan, he has a very sharp mind. I think he a better historian than politician though.
A brief review.
Presidents without a BA — 9, including Washington, Lincoln, and Truman.
Presidents with only a Batchelor’s degree — 24
Presidents with law degrees — 5: Hayes, Taft, Nixon, Ford, Clinton.
Presidents with other graduate degrees — 3: Wilson (only PhD), Clinton, George W. Bush.
Not much evidence for your theory, is there?