smooth end-to-end encryption works only tuta-to-tuta or proton-to-proton
The difference is that proton tries to be somewhat interoperable with other services. It uses standard PGP encryption, you can import public keys to it from elsewhere, and you can download your private key from them if you need it.
* Of course I meant that you can easily export the private key from their web client, which is not really a download as such.