Thickness and illumination. I have both but only one C553. C555 (MOAR-T) being thinner(0.05 vs 0.10) and just the center crosshair illuminated are my preference. My C553 has the complete reticle illuminated (MOAR-30 MOA). The MOAR-20 MOA shows to be just center crosshairs and NF also shows a MOAR-30 MOA-Center Illumination. Not sure if the C553 has these choices or not. Fully illuminated reticle could be distracting to some.