You can view 1960s sales figures at http://www.comichron.com/yearlycomicssales/1960s.html
The only years that Batman, the single comic book, outsold Superman, the single comic book, was in 1966/1967, the height of Batmania. Even then, there wasn't much difference in the sales of Batman + Detective and Superman + Action and Superman had a whole family of titles that were strong sellers.
In 1966, the #1 title was Batman with 898,470 reported circulation. Eight of the next 9 titles had Superman/Superboy in it (caveat World's Finest and JLA also had Batman).
For comic book sales, nobody dominated the market of the 1960s like Superman. Batman, Spider-Man, FF, etc. weren't really close. Archie probably has a better argument than Batman for the 1960s.
FWIW, I do think the idea that the Batman titles were close to cancellation isn't really supported by the sales data. Losing ground and in need of a revamp to stay current in the market place, sure. But, cancellation seemed more a threat to make Kane give up some editorial control than a real possibility because of numbers.