Sigma SD9 has a
3.0MP APS-C (20.7 x 13.8 mm ) sized CMOS (Foveon X3) sensor . On the other hand, Sigma SD14 has a
5.0MP APS-C (20.7 x 13.8 mm ) sized CMOS (Foveon X3) sensor .
Another similarity between these two cameras is that both Sigma SD14 and Sigma SD9 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 SD9 and SD14 sensor size comparison.
Sigma SD9 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. On the other hand, since Sigma SD9 has 35% larger pixel area compared to Sigma SD14, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.