One of the key differences between TACACS+ and RADIUS lies in how they handle the AAA (Authentication, Authorization, and Accounting) functions:
TACACS+:

TACACS+ separates the AAA functions into distinct processes. This means that authentication, authorization, and accounting are handled independently, providing more granular control over each process. TACACS+ encrypts the entire payload (except for the header), offering a higher level of security for the transmitted data.
RADIUS:

In contrast, RADIUS combines the authentication and authorization processes into a single step, which can limit flexibility. Additionally, in RADIUS, only the user password is typically encrypted in the packet payload; the rest of the information, including accounting data, is sent in clear text.