I've only used the Canon 24-105 so can't comment on the Tamron. It's a very nice lens, well, it's an L lens so you'd expect it to be. BUT the only downside to the Canon 24-105 is the F4 maximum aperture. It's fine when you're on the wider angle of the lens, but when you're at 105mm it can be prone to camera shake when the lighting isn't very good.
I've heard so many fantastic things about the Tamron. There's a video on YouTube of someone reviewing it and you could literally see the reflections in someones eye and make out what it is etc so it's a very sharp lens.
Only downside to Tamron is the quality control. Some lenses may be sharp, some may be off so it's always worth while checking to check this.