The tamron 17-50 f/2.8 without VC (the NON-stabilized version) is generally considered to be very sharp. I find that it indeed is very sharp, and useful in low light because of the constant f/2.8 aperture. Zoom ring turns the other way than Canon lenses, its autofocus is noisy but decently fast. I do get some halo-ish glow on high contrast edges with my 17-50 wide open (f/2. at 50mm (i.e. white-black vertical edges), but I do not have enough experience with it yet to tell whether this is a major problem or not (how often/if I'd "lose" shots because of it). Sharpness is otherwise very decent for a lens that cheap.
I've no experience with the Sigma. I only know that this particular Tamron lens stands out because of its high sharpness, big aperture yet so low price. The same Tamron lens with image stabilization (VC) is said to lack this sharpness.
Comparable to 17-40. I tested it vs 17-40 and at some settings it was better than canon. And 2.8.