OAuth = you grant Senly access by clicking a button at Google or Microsoft. Safer (Senly never sees your password), automatic token refresh, and you can revoke access with one click from your Google/Microsoft settings.
IMAP = you give Senly a username + (app) password. Works everywhere, but app passwords expire at some providers (Apple, Yahoo) after inactivity, and you have to re-enter the credentials yourself after every change.
Where possible: choose OAuth. Use IMAP only when your provider does not support OAuth.