Fujifilm X100 has a
12.0MP APS-C (23.6 x 15.8 mm ) sized CMOS sensor and features EXR processor. On the other hand, Sigma SD1 Merrill has a
15.0MP APS-C (24 x 16 mm ) sized CMOS (Foveon X3) sensor and features Dual True II processor.
Another similarity between these two cameras is that both Sigma SD1 Merrill and Fujifilm X100 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 X100 and SD1 Merrill sensor size comparison.
Fujifilm X100 and Sigma SD1 Merrill have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.