I too have had both, a few times, and if I was starting again I'd go for the 24-105.
It probably is a bit softer (IME) but I could only tell by really examining the images at 100% in Lightrom.
I agree with the others that 70mm can be a bit short if you are looking for a one lens solouiton. Yes, everyone is different so it's just an opinion, but considering I use a canon 24-70 MKII as my main lens I feel I can say this, as Ive been tempted to go back to the longer lens.
I do find cropping helps compensate though so it's not really an issue, but if you aren't one to mess about with images too much it's something to remember.
The copies of the Tamron I had were both faulty, but they were sharp. My mate has had one since they came out and he loves his, but it has it's IQ issues as well. In the real world there is nothing to complain about though.
The biggest issue I had with the Tameron's were the slow AF. Good enough most of the time but compared to the 24-105 the AF just couldn't keep up with AI servo. Shooting my son on his skateboard would produce a lot of out of focus shots. All my USM lenses had no issues. I also found AF could struggle a bit in low light.
Considering a secondhand 24-105 can be had for a lot less than the Tamron I'd go for that, you won't loose much if you decided to sell it. Even new it's cheaper than that Tamron.
Just my opinions though, I know lots of people will have others, but after two faulty copies of the Tamron I have no love for it, especially at the price it sells for.