The Graflex had better lenses (having bought in a few top class Zeiss ones), but the higher end Mamiya lenses are close - the bulk of the Mamiya lenses are Tessar type, excellent when stopped down, but rather on the slow side.
Finder wise, I can't tell as my Graflex was topless, massacred by some earlier owner. The Mamiya Universal and Super 23 have excellent finders, earlier Press system cameras are quite a bit worse, though.
Mamiya has quite significantly better backs - the film planarity on the Graflex backs is nothing to write home about, while the Press system "M" backs are among the best 6x9 film holders ever.