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.

One thing no one argues about- XLRs stay connected better than
RCAs. 

Seriously If both products are truly balanced (as opposed to fake balance) then you will get a higher SQ.

I would love to tell you it’s all BS.

In my experience has been XLR cables make a difference in comparison to RCA cables - signal is higher and quieter.

I've also tried different XLR cables - Better Cables, Mogami and Kimber Kable KCTG XLR cables.

I heard a difference between XLRs just as I did with speaker cables.

It may be dependent on the gear. On some components there may be zero difference. On other equipment the difference can appear to be significant.

My Luxman L-590AXII sounds noticeably better with XLR. I don’t have the luxury to compare different XLR cables as I’ve already blown the budget with the purchase of the first set of XLRs in my system. It is likely that a superior XLR may sound even better than the one which I currently own.

XLR is quieter  especially on long runs with better resolution.

I use a couple of different CD players (one with XLRs, (fully balanced), and a few with RCA outputs only.
I get much louder volume out of the balanced XLR connection player than the RCA connection players when they’re hooked up to my preamp. (I could compensate for this on the preamp, but just turn the volume up for the RCA-only players.)

I tried a pair of silver-alloy XLR cables, and the sound was too bright for my tastes.
I switched to an all-copper pair of XLR cables, and it is much more to my liking.

As with all things, you have to pick what sounds good to your ears, and sometimes the cheaper cables sound better than the more expensive ones (case in point above). They do make a difference, though.

I will say this.....I will go out of my way in the future to only buy components with fully-balanced XLR outs on them, (as applicable).

@vanson1

In my very limited experience, I have found XLR to be quieter cables than RCA cables, tending also produce a truer sound, without significant coloration.  With a truly balanced system, supposedly you're not supposed to hear a difference between XLR cables, there are probably few systems that are truly balanced.  While my entire system is not truly balanced, I found that the Mogami Gold shielded cables were as neutral as any.  Some XLR cables that I listened to (which were quite expensive) were just bad souding.