I bought a VTAC sling a while ago, and I wanted to get another one but at this time pretty much every retailer seemed to be out of stock, so I purchased a VCAS.
I got the padded VCAS with acetal adjuster(which just means the buckles are made of plastic, which seems to be pretty tough). It was 50$. I wanted to get padding because my VTAC was chafing my neck a little bit when I used it for a carbine class while wearing just a T-shirt. If you're wearing a vest or something, padding may be unnecessary, but I think it's worth it. It's also more comfortable if you're using a heavier long gun like a shotgun or a bigger rifle. A potential downside to the padding is that you can't adjust the length of the sling as much since the padded part of the sling can't be adjusted.
I like both of them pretty well and I don't regret buying either.
I feel that the quick-adjust works smoother, with a little less friction, especially in the 'tightening' direction.
The VTAC's quick-adjust portion is longer which means the quick-adjust can make more difference. The VTAC can be quick-adjusted much tighter than the VCAS can.
Depending on where you buy, but the VTAC will generally be a little less expensive than the VCAS for comparable models.
The VTAC has a tail which sort of dangles. You can tuck it into a little sleeve up to a certain point. The VCAS does not have a dangling tail. The tail on the VTAC doesn't really bother me anyways.
The VCAS uses the same pull tab for both tightening and loosening the sling. The VTAC uses 2 separate pull points, which is a little more complicated.
The VCAS is longer. This means that it can fit more comfortably over armor and will be more comfortable for a longer barreled gun. That said, it also means when wearing no armor, and using a shorter barrel M4-style rifle, it can feel a little loose, especially if you attach it on the receiver as opposed to at the very front and back of the gun. The VCAS is designed so that even on its tightest setting you can still shoulder and aim it. The VTAC, on its tightest setting will be held very tightly to your body, too close to shoulder it, which also makes it more secure.
Were I to buy either of them now, I would get the padded version, which really makes a difference in comfort for all day carry during a carbine class(or real world use but I wouldn't know about that).
2011 update: The more I think about it, the more I think the VCAS has an edge. The fact that you only have one pull tab makes things a lot simpler. The fact that it won't go so tight that you can't shoulder it makes it simpler, since in any given situation if you were surprised you could still shoulder the gun and fire it. Securing it very tight is not that necessary in most situations that I would personally be in. Also the dangling tail on the VTAC adds more complication and the potential that it might go somewhere it's not supposed to. I read about a few people having it go up in the mag well during reloads. The velcro keeper can mitigate that somewhat, but now you have yet another gizmo to mess around with on it.
All this being said I'm now mostly a fan of single points for ARs.