below are my summary after playing with them for quite some while ago.

Sickma focus real fast but does not guarantee pristine image quality, tammy in the other hand, offers quality instead of focusing performances.

Nikon offers both at the cost of 3 times more expensive.  Canon offers an over priced lens at this FL range.

Reading the topic's title my first thought was.... "When in the world did Sigma release a 17-70mm f2.8 constant aperture lens"

The second thought was "How come I haven't heard of it" 

The answer is plain for me at least : Tammy 17-50mm f2.8 for the constant aperture.

i tested with D70 & D200, on D70 it is pristine sharp while at D200, the lens obviously does not have as good resolved power compare to my N17-55.  On canon, I tested it with 400D & 40D, and I notice the edge sharpness is not as distinctive as nikon variant, the Canon jpeg need a minor sharpening, but then it still obviously sharper than the C17-55IS.  You might wan to test whether ur C Body having mis focus issue with the lens as usual.
p/s: i personally own some of the lenses or have access to it anytime to came to the conclusion.  Not sit at home reading from INTERNET

cheers i dint say the N is cheap   my 1st post already say the N is expensive.  I also dint say N17-55 is newer than S1850. 
My points is, for budget usage the T is worth the price while if u got the money to burn, go for the N if u use F mount lah of coz.
For canon, budget is still the T, but better options would be the C1740 rather than the C1755.

no need to argue....

i owned 17-40 f4L, tamron 17-50mm  f2.8 ( previously ), both lens are sharp!

18-50 mm ... i have no idea..

17-70mm i also no idea..

but any lens can take good picture with smart skill.. :-)

Because i find out that i mostly use F4 and above to shot my picture. besides that, i want the USM and better built lens. So i went for 17-40L.