Sigma Quattro has a
29.0MP APS-C (23.5 x 15.6 mm ) sized CMOS (Foveon X3) sensor and features Dual TRUE III processor. On the other hand, Sony FX30 has a
26.0MP APS-C (23.5 x 15.6 mm ) sized BSI-CMOS sensor and features Bionz XR processor.
Another difference between these two cameras is that Sigma Quattro's sensor lacks 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 Quattro and FX30 sensor size comparison.
Sigma Quattro and Sony FX30 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 Quattro has 32% larger pixel area compared to Sony FX30, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.