Is there any advantage in using raw public keys (RFC 7250) in comparison with pre-shared keys (RFC 4279) in TLS, when I need client authentication and have an unique key per client deployed to them out of band?
I'll assume that you have many clients talking to a single server (having many servers would only tend to exacerbate the differences). With raw public keys, the server has to have the public key of each client handy. Exposure of these public keys may be a privacy violation, but does not compromise the confidentiality and integrity of the communication. With pre-shared keys, the server has to have the secret key of each client handy. Exposure of these secret keys makes it impossible to communicate securely with the clients (unless you have an out-of-band protocol to revoke the key and generate a new one). This gives public keys a huge security advantage.
I do not recommend using pre-shared keys except in a small network where the pre-shared key is generated during an enrollment phrase for very limited client devices on which asymmetric cryptography is unrealistic, and either the loss of a client is not a big deal or all the devices are in the same physical area so a physical compromise of a single client is not a relevant threat.