There are indeed 2 Sigma 17-70mm versions:
1. Sigma 17-70 mm f/2.8-4.5 DC Macro
2. Sigma 17-70 mm f/2.8-4.0 DC Macro OS HSM
The first and older one has slightly better MFD, 0.22m vs 0.20m, and maximum magnification 1:2.3 vs 1:2.7. It is also cheaper.
The second and newer one has image stabilisation and ultrasonic focus motor for all mounts (previous model only had it for Nikon mount).
The Tamron 17-50 has constant f/2.8 aperture, which is a plus. Faster autofocus, slightly better build, smaller and lighter are other advantages. Sigma HSM is more quiet, has closer MFD (0.22m vs 0.27m), and of course 20mm extra range. ISO tests show that the Tamron is sharper in the centre wide open, but softer in the corners.
Which lens is better for you depends on your needs. I could live without VC/OS/IS, but noisy lenses (i.e. without HSM/USM) get up my nerves. Have you also considered the Canon 17-55 f/2.8 IS USM or is that lens beyond your budget?