Generic answer: it depends on how you intend to use the lenses.
Dustin, a member of this forum, has the Tamron 24-70, and it works well for him (see his review threads -- he posts a lot of images). The digital picture also has a review that is worth looking at:http://www.the-digital-picture.com/Reviews/Tamron-24-70mm-f-2.8-Di-VC-USD-Lens-Review.aspx
The general consensus is that the Tamron is close to the Canon version 2 in IQ and signifcantly better than the Canon version 1. I haven't tried the Tamron, but I have used the Canon 24-70 II and can vouch for its IQ and that the 24-70 II does indeed focus and track very well for sports (on a 5D III). I used it for a young boys basketball game, and it behaved like a shorter 70-200 f/2.8 IS II, AF-wise. Some will value VC over AF performance, so it really comes down to how to intend to use it. The Canon 24-70 II is better overall, but is it worth the 1k difference to you?
If I were choosing between the 35L and the Sigma 35 and didn't have either, I'd choose the Sigma 35. It has better IQ and it costs less. But given that you already have the 35L, I'm not sure if it's worth the transaction costs associated with switching, especially if you'd consider a 35L II (if it ever comes out). It depends on how often you'd use it.