Both Sigma SD14 and Sigma SD15 have APS-C sized 5.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another similarity between these two cameras is that both Sigma SD15 and Sigma SD14 sensors lack anti-alias (Low-Pass) filters. 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 SD14 and SD15 sensor size comparison.
Sigma SD14 and Sigma SD15 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.