From all I can see, the USCG 6-pack is mostly a bureaucratic exercise, with little to do with sailing, but perhaps fairly relevant to powerboats.

The RYA Yachtmaster is much more demanding, both si class tiem and on-board examination, and is very sailboat oriented It isa much more respected around the world.

That said, I feel the yachtmaster focuses too much on high competence in traditional navigation, with scant attention to realities of today's navigation where position finding is dead easy, but keeping all the computer stuff running is not.