Just get the Tamron 24-70 f/2.8 VC and you get the best of both worlds.
well.. only when you get a good exemplar.
every good review website reported great sample variance as far as i know.
Just bought the Tamron. Mine is very good. Go to the Shop and check it in the Shop. I tested also the Canon 24-70 and spoke to the sales about variation in samples for the Tamron. He confirmed that they also received variations in quality of the Canon.
I am happy with the Tamron.