I've rented the Tamron 24-70 twice now, and my feeling is it's AF might be a bit slower and more deliberate in it's motion than the 24-105, but the IQ is very good, quite high, and the VC is pretty good. Not sure how it compares to Canon's latest 4-stop hybrid, but it's quite good. I do regret that extra 35mm a bit. Last night I was shooting at WitzEnd, a small music venue, and I was off to the side and the 24-70 didn't quite get me quite close enough, but my 135L was a bit long from the location I was sitting at.
Overall, if Canon were to somehow make a 24-105 or 35-105 f/2.8 IS, and it was at least as good of quality as the Tamron or closer to the 24-70 v2, I'd jump all over it for $1800-$2100 or so. Sure, I can crop in some on the 5d3 at the 24-70, but you lose some resolution on your subject. Shooting for small print or web you can still do that fine, but getting that extra reach out to 85-100mm can be very nice.
However, with the Tamron I can save enough to get me 1/4 of the way or so to the Canon 70-200 2.8 IS v2, so I think I'll be buying it in the next few months. I'll keep the 24-105 around if I'm walking around the beach or hiking, and use the Tamron the rest of the time. The 24-105 is also quite a bit lighter