That's a great decision indeed. When in doubt, go with the tried and true, even if it costs more initially. Over time, you will actually save because you won't have to buy twice.
As for me, I'm a bit more adventurous, so I went ahead and go for the Tamron. I got both, and after about a week of usage, I find that their IQ, and ease of use mostly on par with each other. I have some shots that look better on Canon, and some shots that look better on Tamron. When the focus is dead on, and the subject (or my hand doesn't move), it's hard to tell the difference between the two lens, even for casual pixel peepers.
However, I do admit that the Canon exudes (slightly) better build quality, and a sense of luxury. Is a Merc C class 10,000 better than a well equipped Honda Accord? Opinion will vary.
Are you a pro or prosumer?
Did you consider the new Tamron 70-200 before picking the Canon?
I was and am still thinking about the Canon 70-200 but the more I'm reading and more importantly seeing about the Tamron, I'm thinking it may fill my needs and save me a few $. BTW, I'm not a pro, just love the sport. Whatever I get will be paired with my 6D.
I'm not a Pro (yet), but would consider my self and advancing amature. I did not consider the Tamron this time, though I understand that many have had great success with their 70-200 offering. I don't have anything against Tamron - I have no experience with them and don't have a shop close by to check them out. I opted to go with Canon because I know it will work with my camera site unseen and I have some experience borrowing a friends' mk I. Perhaps that's short sighted of me, but at this point in time, I'm happy to stay with canon equipment.
I am very interested in some of the new sigma lenses that have come out - the 18-35 and the 35mm. I plan on renting those at a later time.