Like Hitman said, the Eotech up forward is for a faster sight picture, etc.
What pics have you seen of CSOR Operators with the C79? All the ones I've seen they have the 553 on their short barrel uppers and Spectre DR on the long barrel uppers.
Difference between the 553 and 557 besides the way it locks onto the rail is size, 553 is shorter, and some other things. The 557 is not in use with the CF and by the lack of NSN not in use with the US Military either. Also I know 553 is used by CSOR - can be clearly seen in pictures.
557 info
http://www.eotech-inc.com/product.php?id=8&cat=1
553 info
http://www.eotech-inc.com/product.php?id=2&cat=1