[exim] GnuTLS and tls_eccurve

What is the preferred way to configure Exim with GnuTLS for desired ciphers and elliptic curves?

Should ciphers and curves be specified with a priority string in tls_require_ciphers and then tls_eccurve left on auto?

Or should ciphers be specified with a priority string in tls_require_ciphers and then another priority string used to specify curves in tls_eccurve?

For example (I know these are not equivalent configurations, but are used as examples of potential configurations):

tls_require_ciphers = SECURE128
tls_eccurve = auto

tls_require_ciphers = NONE:\
                                       +VERS-TLS1.2:\
                                       +ECDHE-RSA:\
                                       +CHACHA20-POLY1305:+CAMELLIA-256-GCM:+AES-256-GCM:\
                                       +AEAD:\
                                       +CURVE-X25519:+CURVE-SECP521R1:+CURVE-SECP384R1:\
                                       +SIGN-RSA-SHA512:+SIGN-RSA-SHA384:+SIGN-RSA-SHA256:\
                                       +CTYPE-X509:\
                                       +COMP-NULL  
tls_eccurve = auto

tls_require_ciphers = SECURE128
tls_eccurve = NONE:+CURVE-X25519:+CURVE-SECP521R1:+CURVE-SECP384R1:+CURVE-SECP256R1