The smaller the pixels the faster diffraction creeps into the mix but with websized images it is likely not noticable. The thing is that FX has less DOF than DX for the same aperture and composition. The reason for this is that DOF is related to 3 things, aperture (bigger is less DOF), Focal length (longer is less DOF), and distance to subject (shorter is less DOF). So to capture the same composition on a FX body one needs to either A) get closer or B) use a longer FL from the same position, both of which shorten DOF. This why those little compact cameras can do so well for macro, they use short FL and which therefore enhance DOF. For macro where the area that you want sharp is 'deep' is might require one to consider options like stacking.

Albert J Valentino
Nikonian Team Member
Vantage Point Images
Mastery of Composition is the Key to Great Photography