A CAC scan is a non-contrast CT scan and a coronary angiography is a contrast CT scan.

They are both ECG gated scans of the heart otherwise so they're pretty much the same scan area and same scan duration, if you're hooked for a Coronary CT angiography you can easily get the CAC at the same time by doing a scan sweep before contrast administration (at the cost of 2 minutes of time and an extra dose of radiation)

Though the logistics surrounding contrast administration makes it a bit more fiddly with a slightly higher risk profile.