RCA vs XLR

Balanced inputs:
An amplifier which preserves separate, symmetrical amplification of both inverting and non-inverting input signals.

A "truly balanced" circuitry achieves the maximum sonic advantages of balanced operation:
common mode noise rejection (CMRR)
and cancellation of nonlinearities throughout the amplification chain.

This being the technical explanation. Theoretically XLR should give best results.

Rest is left to ones ears. Listen and you will know the difference.

Many variables here. If the component is not "truly" balanced, the XLR capabilities are sometimes realized in a compromised fashion (I am told), which could yield inferior sound. In my system, balanced between some components is louder. When overall system gain is higher, perceived sonics can change due to how you have to set your pre-amp, etc.

I prefer XLR vastly over RCA primarily because it offers the possibility of operating with balanced lines. The risk of interference is so greatly reduced that it's the standard in professional audio: microphones, mixing consoles, equalizers, signal processors, tape and disk recorders, etc., all have XLRs and have for decades. XLRs are also mechanically robust and have a locking mechanism.

RCAs are small, relatively inexpensive and convenient, but that's about all that I can say in their favor.

No connector should have a "sound." No connector should audibly affect the audio signal. If it does, it's defective.

I also like XLRs because you can buy great balanced cables from studio supply houses for a fraction of what typical unbalanced cables cost from hi-fi boutiques.