Both are featured in this month's Digital Photo (which conveniently enough I have with me at work at the moment).
The Canon wins the review test:
"the build is first rate"
"the AF is near silent and fast"
"manual focus ring allows for excellent level of control"
Features & Build: 5 Stars (out of 5)
Performance: 5
Image Quality: 5
Value for Money: 4
However, the Tamron came quite a close 2nd to the Canon:
"though the construction is plastic, everything is finished to a very high standard"
"the AF can be loud, and rather mechanical sounding"
"focuses without problems"
"one of the biggest strengths of the Tamron is its manual focus ring - the operation is beautifully smooth and makes focusing a breeze"
Features & Build: 5
Performance: 4
IQ: 5
VFM: 4
The Sigma 50mm f/2.8 EX DG Macro looks to have finished in 3rd place, and definitely the one to go for if one is on a limited budget, and can be stopped down to f/45 on a Canon, Sony or Sigma dSLR.
Hope this helps in some way, and I've been using
www.photozone.de to research my next glass purchase.