ADUC is a subset of ADAC. ADAC is newer and allows for more complete administration. For example, you can enable the AD Recycle Bin, recover deleted objects with the Recycle Bin, and create a Fine Grained Password Policy from ADAC, but you can't do any of this from ADUC.
For day-to-day administration (resetting passwords, managing group membership) probably not. You can do that from both.
ADUC is still very widely used. It's a familiar interface that's been around for 15 years now. ADAC hasn't been widely adopted (yet), but contains more features. If you want to be ahead of the curve and you're just learning now, I'd use ADAC first.