Both Canon 500D and Sigma SD1 Merrill have APS-C sized 15.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
One other difference between these two cameras that is worth mentioning is that Sigma SD1 Merrill's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the 500D and SD1 Merrill sensor size comparison.
Sensor Size and Resolution Comparison image of Canon 500D and Sigma SD1 Merrill Cameras
As seen above, Sigma SD1 Merrill has a 1.2x Larger sensor area than Canon 500D. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Canon 500D
or the
Sigma SD1 Merrill ,
including hyperfocal distance.
