Goodmeme: This is too big and heavy for my liking, I'd sooner keep my 24-70L. I used to have a 28-75 Tamron f2.8, and the size, weight and quality were great. The only reason I got the 24-70L instead was the focus was much faster.
I really hope Tamron remake the 28-75 with ultra sonic motor (USD / USM) and leave off the IS. That will make it into a true competitor to the 24-70 f4 IS L, size wise. And it would probably be lighter and less thick. :)
Err, the Canon 24-70 L is the heavier of the two lenses.
By way of further clarification, I have been using a Canon 24-105 L for some time. A great lens but I wanted greater flexibility in low light shooting.
All my cathedral shots are in available light, usually at 1,600 ISO, but no more than that, with as little image manipulation in Lightroom as possible.
For me, the Tamron lens does the business with super colour rendition.
This lens solved a number of issues for me. I'm 76 with arthritis and am passionate about cathedrals. Both are relevant. I had a Canon 24-70 L which was far too heavy for me and was an ergonomic nightmare when attached to my Canon 5D Mk II; together they were badly unbalanced. Now I can shoot free hand in many locations that are inaccessible with a tripod. I don't know that the lens is there as it is so well balanced.
The Tamron is a cracking piece of glass comparing well with my other Canon L Series lenses. Very sharp and together with Lightroom 4.2 most anomalies are easily corrected. Also, the VC is a real bonus; I've only had one dud shot so far from camera shake. I'm getting some great images.