I believe the sharpness and contrast of both lenses would be equivalent. What yo u get with f5.6 is a lens which is easier to focus, due to brighter image and shallower dof at focussing aperture. You pay for it in terms of $$, size, and weight. For the latter reasons, for field work I chose the f8 lens. Most of the f8 lenses give you a significantly smaller image circle than a f5.6 lens, except for the nikon, hence the recommendation you read.