I think most of what can be said about the Canon 50mm lenses has been said;
50mm f/1.8 II Toytown plastic appearance. No full time manual focus, Non-USM & noisy focus. Surprisingly optically very sharp. Cheap to look at but then again cheap to buy!
50mm f/1.4 Much better build quality with full time manual focus. Micro USM not ring USM and once again is noisy when focussing. Optically very sharp and of course a little faster but prone to autofocus failure? Better background blur.
50mm f/1.2L Built like a tank, is a little bit faster still, has the all-important red ring around the lens barrel (if that might be important) but larger, heavier and much more expensive! A little faster again and the smoothest background blur of them all as you would expect when wide open but shockingly when stopped down not much sharper than the two lenses above it
50mm f/1.4 would be my choice as the middle ground...I have to say that as I have just paid the standard repair charge to Canon of £104 to get the autofocus fixed and need to warrant the outlay!
There is also the 50mm f/2.5 Macro which goes down to half-lifesize but like the 1.8 above doesn't have USM or full time manual focus and the lens barrel extends while focussing.
I for one am still waiting for Canon to release a great 50mm lens that ticks all the boxes but looking at the trending on recent lenses am worried that if/when they do it might be priced out of reach of many of us...
Good luck with your choice!
Kind regards,
-=Glyn=-