For modern typeset documents it’s a close call but I’d say that A4 has a slight edge in that it’s longer and the extra width of US Letter in practice just means

For modern typeset documents it’s a close call but I’d say that A4 has a slight edge in that it’s longer and the extra width of US Letter in practice just means wider margins. But it’s when you start folding and mailing the paper that the A* system really shines, US system is maddeningly frustrating.