I went through this EXACT dilemma a few weeks back (only from a nikon view, not canon) and was comparing a nikon 80-200 with a sigma 70-200 os. I followed some advice on trying out the tamron as well, and I was very disappointed. The af speed on the tamron was woeful compared to either of the other 2, and just something about the build quality (when compared to the sigma mainly) just didn't work for me. Having said that, one common view I've heard from multiple sources is that tamrons are either fantastic or awful depending on the one you get, something I have personal experience of after having to go through 3 17-50 VC's before getting one that worked. So my advice would be to not go grey market but try one from an
LCE or similar near you (or buy from a grey importer where returns aren't a huge pain in the ass) to check it works right, whichever model you eventually settle on.
For what it's worth, I settled on the sigma. As I shoot crop (D7100), I think I chose well.