jlw has summed up the differences well. I have both and use them for different things.
The Canon 1.2 is an excellent low-light and indoor lens, it has a softness and, dare I say it, a "glow"
🙂 at f4 and wider that is very attractive. One example (M6, Tri-X):
On the other hand, it's not nearly so good in bright sunshine, where it tends to very low contrast and some loss of detail, IME. (Bear in mind that these are old lenses, however, and I suspect there's a lot of sample variation between individual examples depending on state of repair etc.)
The Nokton is a very sharp, much less characterful lens, but is probably the better all-rounder in normal lighting. Not too contrasty and excellent micro detail. I use it nowadays whenever I'm shooting outside in sunlight. I still prefer the Canon's low-light performance, however.
A Nokton shot (M6, Tri-X):
If you can't afford both at the moment, I'd start with the Nokton. I started with the Canon and felt for quite a while that it wasn't quite a good enough all-rounder to be my only 50mm, especially as that's my main focal length.
Ian