Hmmm to be honest its not quite that straightforward. At f8 the f4-5.6 is sharpest across the entire image well into the corners, but the f3.5 is much sharper within the main image circle. Both will produce a good image though sharp enough.
That said they tend to vary a lot lens to lens in IQ so its the sort of lens I would heartily recommend you try on your body before you part with your hard earned.