If photography is your job, and you make money doing it and shoot in low light situations where 1.2 totally comes in handy and helps you to create better photographs then the 1.2 version is a no brainer.
If you don't make money with photography and just like to buy great lenses and have the budget get the 1.2.
In pretty much any other situation the 1.4 will do fine.
I had the 1.4, used it for many weddings until the focusing motor died. Had it fixed and sold it to a friend and I bought the 1.2.....it is a way better made lens and 1.2 is amazing and sharp.
The average hobbyist doesn't need the build quality of most L lenses...Or a "1" series body....These are made for people who use them daily for their job as photographers, that is why they cost so much more so they can keep up with the rigors of daily use. They are specialized and offer things like 1.2 apertures for people who need them to perform in low light, give a separation with extreme depth of field and be sharp wide open.