I have not used the lenses before but according to reviews they are both very sharp lenses. Now, do you think you need the IS? If you don't then I suppose you can save the money and get the Tamron instead.
The tamron has some drawbacks... the autofocusing is definately slower than the canon f2.8 (it will be obvious during usage). Another thing is that the autofocusing is rather noisy..."eeeeeeeeee" when the motor moves...
For IS, it is not necessary useful at the wideangle focal length. In my opinion, it is not so important.
Otherwise, it is a very good price if you get a tamron and if you can live with some of the lens' drawbacks.
I think the tamron is pretty value for money.Its a constant F2.8 and much much cheaper than Canon's 17-55IS.IMHO IS is not needed at this short focal length.to be honest,i cant justify paying so much for a short focal length lens;p
i would go the economical way