I have both these lenses and use them about equally. The rigid summicron is extremely sharp stopped down. There's something about the micro-contrast that is almost harsh (in a good way). And the rendering
into out of focus is especially beautiful. It has lower contrast than the lux asph, and is a bit funky wide open. Stopped down it behaves itself, even at f2.8.
The Summilux ASPH is perfect at all apertures, and whilst I'm sure it is as sharp as the rigid, the sharpness seems calmer in a way. Hard to describe! Also it focuses to 0.7m, which is a big advantage over the rigid.
The bottom line is - they are different enough that it is worth having both of them. If I had to pick one, it would be the Summilux, but I'd be very sad to have to!
Untitled by
Silas Slack, on Flickr (Rigid Summicron, FP4)
Untitled by
Silas Slack, on Flickr (Summilux ASPH, Fomapan 400)