Somebody smarter than me ( and that won't take much
) will explain the math better than I can, but the confusion is the Canon is a full frame lens whereas the Sigma is a smaller, DX format lens only. On a DX format camera a 18mm lens is still wider than any 28mm lens, regardless if that lens covers FX or DX. So if you want the "most wide angle" lens for your camera, the choice between the two is the Sigma.
One question to ask yourself, are you ever going to upgrade to a full frame camera some day? If so, stick with lenses - any brand - that cover full frame only.