I'd say, medium-haul routes are your best choice. But, between these two, I would choose short-haul, because operating short haul routes in the 60s is also a good way to earn extra revenue.

On the contrary, long-haul routes are not ideal for the 60s because there isn't an good aircraft for you to fly Euro-Asian or Trans-Pacific routes until 1970, when the Boeing 747 and the Douglas DC-10 comes out.