A Service Account acts as an entirely separate Google account that multiple users have access to sign in to.

Service Accounts have their own My Drive.

A Shared Drive is a shared Drive/Folder only, it does not have its own account, although it can own files and folders.

Shared Drive is a Google-only service that is accessed via an individual's NetID account.

Service Accounts can own files and folders within its My Drive.

Service Accounts are no longer eligible to use Google Workspace storage.

See UW-Madison Google Workspace - Using UW-Madison Google Workspace with a Microsoft 365 Service Account

Shared Drives are eligible for 25GB of storage.

Quota increase requests and be submitted here: Quota Increase Request Form

Service accounts require a domain administrator to add linked members.  See Microsoft 365 - Linking a service account to a NetID

Once members are linked, they will be able to select the service account after they sign in with their NetID.  See UW-Madison Google Workspace - Log in

Accessing a shared drive requires the user to log into their personal Google account that has been added to the shared drive and then to navigate to the shared drive.

Shared drives can be directly changed by anyone with Manager access (Manager access is automatically given to the creator of the drive and can be added or removed by anyone with Manager access).

See Google - Manage a shared drive

Non-UW-Madison accounts/members CANNOT be given access to a UW-Madison service account.

Less secure.

Service accounts are not eligible for Duo MFA, which means individuals logging directly in to the service account to access to Google Workspace are at a higher risk of being compromised. 

More secure.

Shared drives are accessed by logging in to Google Workspace via NetID login, which is protected by Duo MFA.