which lens is the better?
which Canon lens and what are you going to be using it for?
the canon equivalent the 18 -50 is vs the sigma 18-50mm F2.8-4.5 DC OS HSM
which seems the better?
Canon doesn't make a 18-50mm. They do make a 18-55mm f3.5-5.6 IS.
Lens choice doesn't always boil down to brand. Not knowing how you'll be using a lens adds to the difficulty of making a recommendation.
Personally, I'd go with the Canon unless you are going to be shooting constantly in low light where the half-stop of the Sigma might give you a slight advantage. Even then... the Sigma wouldn't be the clear-cut choice.