Hmm, not a lot of love for the 35L here. I have one and I simply love it, I can't compare it to the 50L other than focal length wise. I like the 35mm focal length, even on my 24-105 I find myself often going to around 35mm.
In terms of CA and other, I am not skilled enough (or interested enough to check all details), but I've not noticed anything disturbing in processed pictures.
People are talking alot about the new Sigma, probably for the right reasons. But it is still untested in terms of build and how it will last over time. The 35L was introduced 1998 if I remember correct and as far as I understand holds well over time. I think that is also something to take into consideration. I have one Sigma lens that I used for my old 60D, I haven't sold it because it's not worth anything and the years have definitely taken it's toll on it.
I am sure you will be happy with the 50L though as many of the more experienced photographers here recommend.