Sigma SD10 has a
3.0MP APS-C (20.7 x 13.8 mm ) sized CMOS (Foveon X3) sensor . On the other hand, Sigma SD15 has a
5.0MP APS-C (20.7 x 13.8 mm ) sized CMOS (Foveon X3) sensor and features True II processor.
Another similarity between these two cameras is that both Sigma SD15 and Sigma SD10 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 SD10 and SD15 sensor size comparison.
Sigma SD10 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. On the other hand, since Sigma SD10 has 35% larger pixel area compared to Sigma SD15, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.