I don't own the summicron. I do have summarons though (3.5 and 2.8) and they are (obviously🙂 different than the biogon (that I also own).
I got the biogon only partly because of lower price. I agonized over the summicron/biogon decision and looked at lots of example images. I just couldn't see a substantial difference between them (summicron seemed to have better corner sharpness and less corner CA, but that might be in my head). So, I did what made sense.....and, I saved $$ to put toward another lens (which I still need to find; either a newer summicron 50 or v1 summilux 50, but that's a topic for a whole other thread). For me, the only real selling point for the summicron over biogon is the smaller size of the lens and the focus tab style. I imagine the cron handles much like the summaron 2.8, which I like a lot. I like the biogon handling too, just not as much as the 2.8 summaron.
BTW, the summarons are quite special lenses too. Especially the 2.8. Really, all of these lenses are just wonderful. I'm happy with any of them.