I owned the Canon 135/2-L for a number of years. It is a fine, fine lens, and solidly built, yet fairly compact for a 135mm f/2 lens (I have owned or own three 135mm f/2 models, the Nikkor 135/2 Ai-S ,and the Nikkor 135 AF Defocus Control, plus the Canon L model). I own a Nikkor 85/1.4 AF-D which is a "portrait specialty" type lens, with gorgeous bokeh.
Hard to know how the Sigma slots in: the trend in newer lens designs is for lenses that "test out well" on test charts, with the ability to separate very finely-spaced lines on a test chart being Priority Number One of new lens model design teams; the Canon 135/2 is, first of all, a LOT longer in focal length than an 85mm lens. The 135 compresses the face more, and has a much narrower angle of view behind a person, and also, the 135mm lens literally, magnifies the size of background elements much more so than an 85mm lens does. And last of all: the Canon 135/2-L lens is a legendary imager. It has VERY good contrast when shot against bright light sources. As fast teles go, it is a "small lens", and it conveys the image of a small lens, not a big, fat,wide, "Look at my lens!" kind of thing, which is something 85mm f/1.4 and f/1.2 lenses have been plagued with since the 1980's.
Still: your question can be seen in more than one way. Portraits? What does that mean? Close-in? Half-body? Full-length portraits? Portraits of engagement couples? If you have an APS-C camera, the 85mm length is very handy. For the most-flattering rendering of a face, 135mm is nice on FF. On FF, a 135mm lens makes the world look more "compressed" than an 85mm; an 85mm is more of a naturalistic-looking lens, while a 135mm lens looks much, much more "telephoto-y". A 135 makes the background items look closer to the foreground items, and bigger, and has a very fast transition from the focused zone, to the out of focus zone, even at 20,30,40 feet distant, and this behavior is not the way an 85mm lens draws its scenes.
I dunno....to ME, the Sigma ART line is geared toward high-resolution, very sharp, flat, character-less lens rendering. I am not impressed with the way the ART series looks on people: there is much,much more to a lens than making it shoot a test chart really well. And last: I honestly still doubt Sigma's long-term reliability and build quality, but not the Canon L-series quality and longevity. The ART series has only been out for a few years; the 135/2 L is an old lens design that is well-proven under hard field use, and is seldom ever complained about by owners. I view the 135/2-L as one of Canon's real gems. Buuuut....it is still a 135mm lens, and it might be too long for some users, especially APS-C Canon users.
The issue with primes: making an either/or decision is not the way they are intended to be bought. Primes are part of a SET of lenses. Canon 85-100-135, Nikkor 85-105-135. And so on.