To be honest, I’m not sure of the exact difference. They’re both cleartext; the syntax is slightly different though. With PLAIN, a base64 encoded string w/ both the username and password is sent; like:
AUTH PLAIN dXNlcm5hbWUAdXNlcm5hbWUAc=
From a quick google, it looks like with AUTH LOGIN, you send first the username and then the password (also base64 encoded) together, but as two separate lines.
Both are advertised in the SMTP banner of our mail servers.
250-AUTH LOGIN PLAIN