Does a Canon 24-70mkIII even exist? I've used both the Tamron & the Canon 24-70 (mkI) on my 6D and I wouldn't trade the Canon for the Tamron for any amount of money. The basic problem in my opinion is AF speed and accuracy, as it almost always is with 3rd party lenses. The Tamron is pretty damn sharp and would be a better bargain if you don't mind missing AF now and then or having it hunt. I shoot gigs, and like weddings, light can be low and there's many moments that you can't afford to miss. If you feel the same, I'd go with the Canon. You won't regret it. Your bank balance might, but YOU won't.