i have read the cqb website until my eyes are crossed and don't see the difference between the sop and cst.   it looks like the sop has the transition release buckle so does the cst.
what is the main difference between the slings.

The SOP is 1.5" wide to better distribute the load with 1" attachment straps to be used at each end of the sling to interface with the mounting adapter sling loops or sling swivels. This allows it to move, shift and conform within the sling loops to allow the SOP Sling to move more efficiently with the weapon thus enhancing free movement of the weapon and allowing the weapon to hang in a more vertical orientation while being used in the Military Patrol Ready carry mode

The CST is 1.25" wide.

Other than that I think they are pretty close to identical…

I Just picked up the SOP from brownells this morning.  I have previously used the Blackhawk swift and the Eagle Tas 3.  The Eagle is very similar to the SOP but just putting it on my rifle I am very impressed.

The SOP, being wider, is the better long term carry / field sling.  

The drawback is that the 1" webbing attachments allow the rifle to flop around when used in the muzzle down 'off side drop' carry.  The CST keeps the butt of the rifle in tight to your chest in this carry mode.