Please accept this as a genuine question. Are you able to point me to any examples which show the difference you are talking about between the Art and Canon lenses?
I so often read things on the internet claiming the Art series lenses have poor bokeh, but I rarely see photos shot side by side of the same scene, etc, to provide a good comparison of an Art lenses with a comparable lens - and when I have, I haven't felt convinced the criticism of the bokeh of the Art lenses is justified. For example, this website
http://willchaophotography.com/sigma-50mm-f1-4-art-review/ has comparison shots taken with the 50 Art and the 50L but it doesn't convince me the 50 Art's bokeh is poor (and in fact the author's opinion was it is not). When I look around on the internet at images shot with the 35 Art and the 35L II, I haven't seen anything to make me complain about the 35 Art's bokeh either - but I'm not sure if I've ever seen sets of images taken under the same conditions providing a good comparison between them. (I've seen test shots comparing things like sharpness, but I'm talking specifically about comparing bokeh.)
I do sometimes think the 85L II at f/1.2 may be able to produce slightly better bokeh (as subjective a concept as that is) than the 85 Art, but I put that down to its wider aperture, as a I think the same about the 85L II and any 85/1.4 I've seen images from. Again though, I'd like to see good comparison shots (taken under same conditions, etc) to really be sure.
Anyway, I'm not someone who values sharpness above all else in a lens - I am interested in the overall look of the image produced (again, however subjective a concept that may be), and sharpness is obviously only one factor - so if you can point me to any good bokeh comparison shots, I'd be genuinely interested to have a look.