The biggest difference would be the theoretical claim that proton can’t know anything about your emails because the mailbox itself is encrypted. The calendar too. This also means these accounts aren’t compatible with any IMAP/POP3/Activesync clients, and you need to install your own proton plugin to use it with them. On the desktop. On the phones they have their own apps, since you can’t use the phone email app nor the phone calendar. They are a bit lacking there too. Regarding the mailbox theoretical encryption claim, I’m sure it’s really encrypting everything. It’s just, email is inherently unencrypted (unless it’s proton to proton) as it travels along the servers, unless you go to several pains to encrypt it, and your destinatary too, to decrypt it on their end. So for most purposes, right now the main difference between these two doesn’t seem all that useful and it continues to be relatively simple to intercept/read your email along the way, since most likely it won’t be encrypted anyway.