XLR or RCA cables?

Short answer:
If equipment supports the AES47 standard, the the cable will make less difference-compared to RCA.
I suggest you PM Ralph Karsten (Atmasphere) for a complete reasoning- though I think it is on his website.

As far as personal experience goes, I bought some cables from Ralph when I bought his amps and preamp. I think it is Mogami.
I  though I was done with it, but I tried some AQ cables (Mackensie?).
To be honest, I couldn't hear a significant difference between the two.
Bob

I have seen where a manufacturer will recommend XLR if you are making a long run.
I have both but my runs are short and I can not hear a difference.

If you are connecting truly balanced components then XLR is preferred....if not balanced then buy all RCA you can and listen for your liking.  Balanced cables sound very much alike.