If a person doesn't own a 50mm lens and they want a 50mm prime (usually the only reason you'd want one is for the lower focal ratio since most working zooms already cover the focal length -- just not at a low focal ratio) then it probably makes sense to sort through the decisions.
In my opinion the 50mm f/1.8 II was really _only_ for those who had very little budget and wanted something. It's advantage was it's price tag... that was pretty much it.
The "reasonable" lens was always the f/1.4 with more aperture blades and a better bokeh quality (just not nearly as good as the f/1.2). The f/1.4 was better than the f/1.8 II in literally every way *except* the optics of in-focus details were extremely similar (the f/1.8 always did have good optics of the "in focus" areas and it was mostly the out of focus areas were it had a very poor bokeh quality.
BUT THEN... Canon updated the 50mm f/1.8 to an "STM" version, improved the build quality, improved the aperture blades, which in-turn improved the bokeh quality and yet the STM lens is STILL extremely affordable.
The f/1.4 picks up 2/3rds of a stop of light... so there is that. The f/1.2 picks up yet another half stop of light (or about 1.3 stops faster than the f/1.8).
But given the OP already owns a 50mm... it all makes me take a step back and wonder why a decision is even worth of consideration at all.
Do you use your 50mm a LOT?
Is there something about your current 50mm that's just not meeting your needs? Do you look at your current images and think... boy I sure could use something that gathers more light to let you cut back on my ISO setting and reduce noise? Or maybe you're thinking that f/1.8 depth of field is far too broad and you want something much much shallower??
I own 7 different L series lenses but I confess that I do not own a 50mm at all... nor do I own an 85mm at all. I take most "portrait" shots with my 70-200mm f/2.8L IS USM and I love it. I also own the 135mm f/2L USM and while I love that lens too... the 70-200mm is just so goshed darn gorgeous in image quality while also being so goshed darn VERSATILE that I found only a few situations where there's a compelling enough reasson to pull the 70-200 off the camera body to attach the 135mm f/2 (and hostly the 135mm f/2 gets used more for low-light concert photography than it does for traditional "portrait" photography.)
So if this is really a matter of "I have $1500 that I could potentially spend and I'm wondering if the 50mm f/1.2L is worth it" then I'd spin the question around and ask... WHAT ELSE might you get $1500 worth of value out of?
What sorts of things do you shoot with your 50mm and where are your pain points where your current lens is leaving you wanting more?
Tim Campbell
5D III, 5D IV, 60Da
