Never tried the Sigma 35 but I had enough headaches with other Sigma lenses.
The 35L is the most consistently focusing lens I own or tried, and this includes 20+ lenses, at least half of them L (most of them and in the "tried" category).
Some people, me included, have concerns about the bokeh of the Sigma vs. the Canon. The 35L can often render harsh background as well. It would be interesting to hear your opinion after you get the 35L.
I'm curious about your statement about the 35L rendering. I've been using my 35L for about 6 years now and it's always been creamy and very nice when shot wide open. Sure it needs critical focussing and it's AF isn't that great in really low light. But the rendering is really something special. I use mine alongside an 85L for weddings and they are easily my most used lenses. The two lenses have a certain look which compliment each other.
I've said everything I'm going to say about the problems I've had with Sigma over the years in other threads. The 35L is a very old (but still capable) lens. But it's nearing the time where a mkII is needed. Better coatings, weather sealing, new AF design and slightly better optics come to mind. Will the mkII cause me to rush out and upgrade my mkI? Probably not right away. Will the mkII make better sellable pictures? Probably not. Will I get a Siggi over the 35L....certainly not
I'm a hardcore Canon L fan... Have 20 L lenses, never wanted to buy anything else, but let me tell you this Sigma one is something has totally surprised me.
Have you seen the lens image quality tool comparison with the Canon one at the-digital-picture.com ?
Wide open the Sigma is much better than the Canon not only on the corners but the center too. And the Canon have quite some fringe... Even down two 2.8 you can see a nice rainbow on the thin lines on the Canon and very minimal on the Sigma
There has to be a point where a lens performance is rated a good enough, more than adequate...a great professional tool. This is where I see the 35L. I seriously doubt that my customers will see any real world difference in the sharpness but a heck of a lot of difference in the colour balance and rendering. I stick to one lens brand so that my pictures carry the same colour balance too. I don't see how any hand held shot using a 35L or a Siggi 35 f1.4 is going to make a lot of difference. Sure....pop it on a tripod and shoot a lens chart....we all make sell loads of pictures doing that