VCAS - hands down
I dont like the VTAC due to the tail can get hung up when the sling is tightened -- no lose ends on the VCAS -- and now they make a padded version for those who dont run around in Armour.
SOC-C is a great sling if your using a bunch of different weapons -- I've used one on a M4A1, Ak-47, CZ-58, PKM, RPK, and M249 para.
But too much (IMHO) for a dedicated weapon setup.