If we are just talking purely about sharpness, there are a few things to take into consideration. It is not as simple as one being sharper than the other anymore.
I can tell you right now that between the various systems I have shot in the last two years (a7r/551.8, 6D/5D3 with various high quality lenses, EOS M with 22/2, Fuji xt1/23 & 56mm, and pentax k5IIs with 50/1.8), the sharpest (and most consistently sharp) results I have gotten have been from the Fuji and Pentax rigs. Oddly enough, both of those happen to be APSC.
You have to consider the AA filter (strong, weak, not present), pixel density and how it affects your shutter speed requirements, glass, etc.
If we are talking other things like varying resolutions (ability to zoom in), DOF, etc, we are no longer talking solely about pure sharpness at average/normal viewing mediums.
Again, what I have referred to are the results I have experienced for myself so YMMV. But I know that I am not the only one who has used all or some of these systems and come to the same conclusion.