This question is relevant for every generation of Instax Mini, which has a pretty regular update cycle. In truth, there's no appreciable difference in image quality among any of the Instax Minis released in the last few years; the lenses and flash units stay virtually identical (as, obviously, does the instant film).
The main differences come in the aesthetics / ergonomics, as well as the additional features on newer models. In this sense, the Instax Mini 12 is better because it possesses parallax correction – making it easier to compose images in close-up mode. But otherwise the Mini 11 produces photographs that are every bit as good.