For portraits, I shoot them both !
The mid '50s Summicron lacks the hot Sonnar look,
and has an across the field clarity. Wide open, it hints at soft tonality.
If I want to draw peach fuzz on a young cheeks, the Nikkor.
Lighting falling on the contours of a ... more mature beauty, Summicron.
Subtle, subtle, subtle. A lot like a mid '50s Planar on your Rollei.
The only way to tell for sure is to shoot them side by side.