CBS (Cell Broadcast Service)
First, you would see high level difference as follows :
More importantly, CBS and SMS/USSD are transmitted by completely different channel and mechanism. Whereas SMS/USSD is transmitted by DCCH->DPCH as normal signaling message, CBS is using its own independent channel (CTCH-> SCCPCH) as shown below. (Channel Configuration for CBS is configured in SIB5 or SIB5bis)
CBS Message parameter and an example is shown here.