If your goal is portraits then I think the Tamron would be nice having 75mm reach (almost 80mm- great portrait length). If you were into landscapes, I'd say definitely go with the Sigma to get 24mm on the wide end.
As much as I like the Sigma 24-60 it has some faults, such as slow focusing (no HSM) but I do find it accurate most of the time. No full time manual focus. Suffers from zoom creep if the lens is pointed down, but it does have a "lock" switch to keep it at 24mm when walking around. Compared to lenses like the 17-55IS or 24-70L it is much smaller and lighter, which is a good thing. And again 60mm on FF can be a little short for portraits.
The absolute most fun I have with my 5D is with my Sigma 50mm f/1.4 lens. Thin DOF, HSM focusing, sharp and overall great IQ!