I think a Summicron is something everybody should try if they have an M Leica. The point is you will never know for sure if you don't. I would recommend a newer rather than older lens, so you can at least experience the maximum available from a great lens at all apertures. Older Summicrons are less contrasty and not so well corrected, which can be good, but for that sort of look I would say a Summitar is king.

My Summitar is a 1946 coated example with a coating mark on the front element that I don't believe makes any difference to the image. But it has an amazingly weird yet attractive bokeh wide open, and gives a really nice gentle look to modern contrasty colour film like Ektar. It may or may not be 'better' than the Nikkor (I think a later Summicron would be) but they are real character lenses. Get a cheap Summitar-to-39mm adapter from Ebay to use modern filters and lens hoods.

Steve