Sony W550 has a
14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features BIONZ processor. On the other hand, Sony WX220 has a
18.0MP 1/2.3' (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Bionz X processor.
Below you can see the W550 and WX220 sensor size comparison.
Sony W550 and Sony WX220 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 Sony W550 has 28% larger pixel area compared to Sony WX220, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.