I had a Summicron and a Biogon and kept the Biogon. You can buy a Biogon brand new for less than a used Summicron these days. The performance of the Biogon is pretty spectacular too. It is practically optical perfection. I took a lot of the same images with both lenses and the Biogon was crisper, had more even illumination and was better corrected for distortion.
There are a lot of choices and the reality is you would be fine with almost any of them. As they say, so many choices, so little difference.