In every imaginable parameter the XLR is superior to the RCA connector.
It's all about the implementation. I've seen some really horrible XLR connectors that I wouldn't want anywhere near my system. On the other hand, I've seen some really exquisite RCA connectors - some of which even lock - from manufacturers such as WBT. So there is good and bad quality of each.
But that's not the question. Is it audibly superior at lengths less than 5 meters? The answer is no.
I think it unlikely you'd be able to reliably hear differences between XLR and RCA terminations provided each are well built. If the XLRs terminate into balanced connections, you may very well be able to hear and measure differences. For example, many balanced components (such as Audio Research) will have 6 dB greater gain on the balanced connections. If you don't need that gain, you might want to consider that as 6 dB better S/N. Either way, that's a substantial difference and cable length has nothing to do with it.