It really depends on your environment and your use-cases. Our team was recently faced with this same issue and opted to continue with vCD since we already had a working instance that services customers. One of the things that you need to know is that vCAC out of the box provides very basic functionality, so in order to do some of the really neat things that vCAC offers - you need to incorporate vCO.
- vCAC does not allow resource 'throttling' like vCD does, so if you charge based on a specific vCPU speed (which we do), you need to tweak your chargeback model or stick with vCD
- vCD will be end of life 2017 so if you are going to vCD route, it might be worth while to look into VSPP which will continue support and development for vCD service providers only.
These were just a few things that we came across when making this decision, so I figured I would share. vCAC is a great product and as you mentioned seems to be the strategy of VMware's future, but in our case we are waiting to see what the next version of vSphere has to offer since some of vCD's features should be moving down the stack to vSphere.