The Z and H ratings are speed ratings. It just means that the tires will be fine up to a certain speed (or in the case of the Z rated tires, past a certain speed). Oftentimes, the higher the speed rating, the lower the mileage rating. It can be assumed that this means the compound is softer and further assumed that they'll grip slightly better. I don't know how noticeable the difference is though within a same type of tire. Anyway, I'd say the 712s in the Z rating are perfectly fine for daily driving.