What is a better choice of lens. 70 - 200?

Tough choice, what are you going to be using it for? If you are a beginning photojournalist or wedding photographer and are going to need that f/2.8 then I say go with the Tamron regardless of images quality because some softer images are better than no images. If this is just a recreational use lens then I would say go with the Canon because the build quality is far superior, the value will hold if you take care of it and ever want to sell it, and the images will be far sharper.