Probably the biggest difference between buying a genuine Canon lens or an alternative brand is resale value.
A good Canon lens will hold its value, whereas third party leneses do not.
Also, you have to consider the build quality and reputation, and I think that Canon has that in spades.
Tamrons are usually considered to be "cheap" lenses.
I'm not saying they are bad, as I own a few third party lenses that perform very well, but if I went to sell them, I wouldn't get anywhere near what I paid for them.