Upload & Sell: On
I just picked up a used 24-70 mark i literally 2 days after I decided to pull the trigger on the Tamron. The Canon just landed on my lap (a deal I really couldn't refuse) and its absolutely perfect in every way.
Outside of VC - which IMO - is really useful for video, there's no comparison between the optics of the mark 1 and the optics of the Tamron - at least my copies anyway - (I went through two Tamrons - the first back focussed out of the micro adjustments ability to correct it on a 5D Mark iii) and the 2nd one - which focussed much more accurately - still wasn't nearly as sharp as the Canon. Now I recognize sharpness can vary from copy to copy - same for Canons - but the 2nd Tamron is going back tomorrow. The Tamron is a solid piece of glass, but at f/2.8, at least for me - the Canon is noticeably sharper...
BTW, if I were a videographer who used DSLRs, I would get the Tamron - the VC makes a HUGE difference in video where optics - though important - aren't nearly as important as they are with stills....
Someone else mentioned AF - the AF of the mark i is noticeably faster than the Tamron....