both are fantastic lenses!
here are some thoughts:
Sigma has a faster, quieter auto focus, but I am assume you will not be using that.
The Tamron is actually slightly sharper than the sigma and even (surprisingly, slightly sharper than the canon (according to Popular Photography)).
The build quality of both are good, but the sigma is a little stouter.
All in all though, I would recommend the Tamron for video work and occasional photos and the Sigma if you are doing any major photo work, such as sports, as well.