Both Sigma SD15 and Sigma DP2x have APS-C sized 5.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another difference between these two cameras is that Sigma SD15's sensor lacks 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 SD15 and DP2x sensor size comparison.
Sensor Size and Resolution Comparison image of Sigma SD15 and Sigma DP2x Cameras
Sigma SD15 and Sigma DP2x have the same sensor sizes so they will provide same level of control over the depth of field when used with 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
Sigma SD15
or the
Sigma DP2x ,
including hyperfocal distance.
