Both Sigma DP1 and Sigma SD14 have APS-C sized 5.0 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 SD14'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 DP1 and SD14 sensor size comparison.
Sigma DP1 and Sigma SD14 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.