I have had both until recently. The rendering is different, Paul. I think you know the ASPH Lux is sharp at all apertures, and rendering is neutral with smooth oof areas. The rendering of the Nokton is sharp but not as sharp as the asph lux, particularly as you open it up. The nokton gives more of creamy or dreamy rendering -- you'd love it in colour -- with the edges of the frame being slightly less sharp than you'd see with the asph lux. I'd call the nokton a warm rendering lens with beautiful, impressionistic oof areas. The nokton does not have a flare problem that I know.
All said, the nokton is a very large RF lens and that affects whether or not it stays or goes on certain days/nights. The nokton was as big as my big bertha lux 75! Is that going to strain your usage...of course, you need to answer that for yourself.