And already started searching web for 50 1.2 and cannot decide if ai or ai-s.
They are virtually identical.
Same optical formula. Same coatings. Same results (I made some extensive tests), except for the OOF dot shapes when stopped down: 7 vs. 9 straight blades, but in this particular case it does not really matter.
Externally, the AI outer diameter is larger by 2mm and it looks more like a Noct, while the AIS is a bit more "cylindrical".
Rendering, at medium apertures, is quite vivid - that elusive "3D pop" effect seems to apply, but is probably subjective. This is why I often use it for sreet shots (esp. demos or festivals), first because the FL is adequate, second because I get lively scenes despite unpredictable and/or messy surroundings. Clean images at f/2 also help. f/1.2-f/1.4 are more for the fun; here the Noct is definitely superior, but I do not want to put it at a risk!
I guess there is more variability across copies than across lens types (AI/AIS) here, so look for the best deal.