Do you already have the Franiec grip? If not, then why not just get the Sony, especially since it'll be covered up by the case?
I don't use a Gariz half-case (or any other) with my RX100, but I have the Sony grip on it and it's nice. I've never used the Franiec grip so I have no basis comparison, but the Sony grip is definitely more than good enough, at least.
And I have a Gariz half-case on another camera that needed some additional grip-point, and love the case. Better ergonomics, better aesthetics, and a bit of extra protection.