Seems like this round gives you the option to get the super cheap ABS plastic case instead of or in addition to the aluminum one. As for the differences between the gk62 and gk64:

The gk64 is standard layout except for the 2u left shift and a 1u right shift. That left shift key is getting easier to get with more and more keycap kits. The 1u right shift you could use any 1u key for it if u don't have the shift specifically. Everything else is standard.

The gk62, however, has a regular left shift, but 1.25u arrow keys , ? key (instead of 1u) and a few others.

I believe the pcb is the same and supports either layout, but I could be wrong there.