Michael,
According to Photodo testing. The two lenses are quite equal in contrast (0.91 vs 0.93), but Contax wins in the resolving power (0.62 vs 0.70). The sharpness is somehow subject to individual preference or eyesight. I personallly do not believe there is much difference between the two except for those subjective feeling's. Who can tell with normal bare eyes the 10, 20 or even 40 line pairs per millimeter? Who even cares the difference down to micrometers for circle of confusion if he is not to enlarge a half wall print?
I once made an 12" by 18" enlargement with Fuji Reala (claimed the finest negative) taken with Planar 45 with tripod in good light. You guest what? I need 'interpolation' (of the laser printer from the lab) to keep the color saturated. From this, I learn that use the right tools to do the right things. Don't go into the extreme!
Nikkor AF 50mm f/1.8 Effective focal length: 51 Weighted MTF for 50 mm: f1,8 0,62, f2,8 0,78, f4 0,83, f8 0,86 Average Weighted MTF: 0,85 Grade: 4,4 Weighted MTF 10 lp/mm: 0,91 Weighted MTF 20 lp/mm: 0,82 Weighted MTF 40 lp/mm: 0,62
Contax Planar 45mm f/2.0 Effective focal length: 47 Weighted MTF for 45 mm: f2 0,71, f2,8 0,83, f4 0,88, f8 0,87 Average Weighted MTF: 0,87 Grade: 4,7 Weighted MTF 10 lp/mm: 0,93 Weighted MTF 20 lp/mm: 0,85 Weighted MTF 40 lp/mm: 0,70