I have had bunch of 50's for the Leica. Two Sonnar 1.5s, one post war and one prewar, Russian stuff, and a converted Pentax-M 1.4. I used to have a mint Summicron, I think it was the first version of it (same time period as my 1000000 M3). I tested all the lenses I had a few years ago and surprisingly the prewar 1.5 uncoated Sonnar bested the Summicron in resolution, and beat the post war Sonnar too. It is probable that I have an incredible version of it because it didn't seem logical. I ended up selling the Summicron, although it is a good lens, because it made no sense to keep. Fast forward a few years and the 50 that I carry all the time now is the converted Pentax lens. Fast, sharp, wonderful tonality. Possibly the best all around 50 ever made, although on the Leica it focuses the wrong way.... I have been using the Pentax for a few months now and it is such a good lens I am thinking about selling my Sonnars.

If you are choosing between a Contax Sonnar and a Summicron the choice is not so simple. The Summicron will flare less but isn't as fast. You probably won't be able to tell the difference between them outside in bright sunlight stopped down. If you want a lens for low light, keep the Sonnar. Summicons are worth at least 5 times as much. One thing I liked about the Summicron is the negs had an "etched" quality about them.

If you are shooting color, I would sell them both and get a modern Planar.