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.