RCA vs XLR

Ideally, neither XLR or RCA plugs should have a characteristic "sound". Pro audio types use XLR because of its relative immunity to RF contamination and its locking connection. In a low RF environment it's possible that single-ended RCA connections may sound better than balanced XLRs. This is due to the fact that truly balanced equipment requires a much more complex circuit design and implementation. In other words, it's easier to screw up a balanced design than a single-ended component. That's in theory, in practice it may be just the opposite. I use balanced XLRs, but then again I live in a high RF area. Your dealer is at the very least overly simplifying the issue.

In a truly controlled (lab) environment with unlimited
budget design XLR will be better. Real world it isn't that
easy with some maufacturer's RCA sounding better than some
other's XLR. Even if it means spending a bit more, always
test your targeted componet in your room with what other epuipment it'll be connected to. Let your ears and taste
make the final decision.
it's going to be connected to

Thanks Brauser for your sensible post. XLR against RCA is neither a matter of faith nor of simple specs and "science" generalised. The proof lies in the listening! At least that has been my experience too. An afterthought: Balanced power lines, be it through Equitechs or the PS Audio Power Plants, I personally found a true revelation in the general lowering of the noise floor. Got nothing to do with the topic of this thread, was just thinking about "balanced".

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.