But the conclusion I've come to... is that Canon may cost more, but more often than not, it is well worth it.
As long as Sigma has no choice but to reverse-engineer Canon's AF system, there always remain a chance that a Sigma lens may no longer work, or no longer work 100% (such as weird focus behavior in live view, etc, which I have experienced with some Sigma), with future Canon firmware updates or future Canon bodies in general.
Having dealt with Sigma service before with headaches abound, I can understand those who stick with the Canon brand.
In addition, unlike the Sigma 35 1.4 ART and Canon 35 1.4 situation, the 50 1.4 ART vs. Canon 50 1.2
L situation do not deal with the equivalent lens aperture class. I understand many here who will say the small stop of light advantage does not mean anything, especially since it's not sharp wide-open, etc, but the general public may feel differently in perception.
Having said that, I will happily keep using my 50L for another year or so, as I do love the small size and rendition of 50L, then I will likely pick up a used 50 ART or on heavy sale in future.