I haven't seen the Canon 1.2, but if it's anything like the Pentax f/1.2 then it's considerably larger and heavier than the 1.4 or 1.8.

The nice thing about the f/1.8 (1.7 or 2 in Pentax mount) is they are tiny, cheap and very very light. There isn't even a space related reason not to carry one.

My Pentax 50mm f/1.4 is the only lens I always carry, there's just no reason to leave it at home. (at the time I bought it, Pentax sold their 1.4 or £160, so it is price wise somewhere in between the Canon 1.8 and 1.4)