456 is closer to what the machine was originally designed to roll with, which is a good thing. Its also a pretty stable, good sounding formulation.
GP9 sounds best when printed hot, which an ATR is certainly capable of, but it's not a free lunch. There's bleed through, and inherent issues that come whenever one works outside the original design parameters of their gear. For some styles of music, the non-linearities might be desireable, and it can certainly improve s/n overall. GP9 might be great for some projects, and even as a good effect, but i'd use 456 as a default, and work diligently to control s/n end to end.
-d-