They're both very good performers. I have both, though I've never done head to head testing.
The Serenar is significantly more compact (e34 vs. e39) and maybe slightly heavier, but not as nicely built, IMO. The Summaron moves much more smoothly, and the ergonomics of the focusing and aperture rings are better.
The Summaron (at least the M-mount version) focuses down to 0.7m, while the Serenar stops at 1m.
For hoods, I like the IROOA on the Summaron and a FOOKH on the Serenar (much more pleasant than the original Canon hood).
The Nikkor 35/2.5 is smaller than either (maybe too small), but it has a single rotating barrel like an LTM Elmar, and takes impossible to find 34.5mm filters.
I enjoy using the Summaron more than the Serenar or the Nikkor, but any of the three will produce great results.
Ari