Tamron 24-70, no contest. Fantastic lens, IS, and with cash you save over the WAY too expensive Canon you can buy a couple more toys or go on a vacation. I've read every test and review I could find, there's nothing the Canon does better that would be worth that much cash, at least nothing I can imagine. For the money saved you can buy a great flash or two or a Sigma 85 and a flash and so on. Canon missed the boat here IMO and not adding IS is almost rude. Yes, you can crank up ISO, but you can't crank up ISO on the fly, I prefer my lenses to help out here
Go see if you can try it at a store somewhere, I could not be happier. It's my first non Canon lens, but probably won't be my last (Siggi 85...)
Great suggestion. The Canon 24-70 II is sharper and has less distortion at 24mm than the Tamron, but the Tamron is still excellent at 24mm and outperforms the Canon at 70mm (in the corners). It all depends on what you need. For my needs, I wanted maximum performance in the 24-35mm range, so I went with the Canon 24-70 II. The Tamron, though, is an excellent value; well-worth the OP's consideration.