Sony T99 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 TX66 has a 18.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features BIONZ processor.
Below you can see the T99 and TX66 sensor size comparison.
Sony T99 and Sony TX66 have 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 T99 has 28% larger pixel area compared to Sony TX66, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.