Whatever you do, buy 2nd hand (off eBay etc..) and sell when you don't like it.
You'll pretty much get your money back.
As I said I used to have the Tamron 17-50 nonVC, and sold it a month later for pretty much what I paid for it.
I also got a few primes (2nd hand off eBay) as well as the Tamron... the older nonIS versions of the EF 24, 28, and 35.
All these easily beat the Tamron hands down.
I soon realised 28mm was "right" for me. It became my main lens.
I sold on the 24mm and 35mm (again for what I paid)
When the new EF 28 IS version came out, I got one.
Now it's my main lens.
If I want a zoom I'll use the 18-55.
The Tamron is not all it's hyped up to be, and far too many dismiss the 18-55 because it's a "kit" lens and so (wrongly) think it must be cheap & nasty.
Better/expensive lenses DO NOT equal better pictures.