That is a big question. The obvious answer is...get them BOTH.
If that is not an option, then you need to decide your intended purpose. Do you need a 52" bow for a specific purpose? (i.e. ground blind or tree stand clearance?)
I love my Grizzly, but I have not had the pleasure of shooting a K-mag. I have heard various opinions on them, most of which deal with stack, or its uncanny lack thereof despite its length.
If you are in a position to shoot them both, pick the one you shoot best with, ..right?