XLR or RCA?

Noise gets in by direct capacitive coupling or electromagnetic pickup. In case of capacitive coupling shield can be very effective. Electromagnetic pickup is a little more complex. Shield is made of aluminum or copper - a non magnetic material and cannot protect against electromagnetic waves, but because of skin effect induced noise currents flow on the outside of the cable - shield. Skin effect works at higher frequencies but at low frequencies cable becomes effective antenna when length of the cable approaches 1/10 of offending signal wavelength. It means that if your cable is very long it will pick-up all sorts of electromagnetic interference in spite of shield. Balanced cable not only allows to reject common mode noise by being balanced but also by twisted wires inside - very effective against noise pickup (capacitive or electromagnetic) by exposing both wires to fields evenly causing same noise currents in both conductors producing cancellation. Keeping cables short is extremely important. My IC is 0.5m XLR.

Fully balanced signal is what makes live sound reinforcement buzzless and clean (except for electric guitar players with noisy single coil pickups)...with 100 foot (or more) snakes. You can take an unbalanced signal cable from a live musician's instrument pickup, stick that cable into an active "direct box" that changes it into the balanced signal the mix needs, and the result is clear as a bell. Also, the "future" is wireless live mixing (use an iPad).
When I play electric guitars I end up using long runs of unbalanced guitar cables (all the stomp boxes with an input cable and out to the amp) and have to apply small "micro amp" boost in there to unload the pickups and retain a modicum of fidelity. Nothing quite like a modicum of fidelity.

I used to own the McIntosh MC275 amp. Even though it is not fully balanced, I tried both RCA and XLR cables (Crimson Music Links) between it and my preamp at the time, an Aesthetix Calypso. I preferred the sound with the XLR cables -- I thought my system sounded fuller, with better bass than with the RCA cables. This probably had more to do with the Calypso, which is fully balanced, than the MC275, which is not.

In any case, why not just borrow XLR cables from the Cable Company or buy them from Music Direct, which offers a 30-day money-back guarantee? If you hear a difference and like it, then buy the XLR cables. If not, return them.

OK, in theory from this discussion should I take it that, all materials being equal, XLR cables are more resistant to noise than RCA cables? That noise can be both a function of cable length and EMF environment? And furthermore should I assume that more complex construction, at least in terms of shielding, should reap bigger benefits for RCA cables versus XLR cables? But what about wire configuration (I.E. metallurgy, thickness, shape) and dielectric composition? If you believe these are relevant at all, wouldn't they be relevant and affect the "sound" for both types of interconnects?

I understand and accept that many of these variables are highly application dependent (gear, physical location relative to EMF, ears and beliefs of the listener), and there is no substitute for "try before you buy", but I am just looking to see if I have grasped the relevant information in the discussion above...

kn

Yes, XLR is more resistant to noise. Shielding plays bigger role in RCA since XLR has inherent noise rejection by usage of twisted pair. Materials still plays role (capacitance, dielectric absorption, metal purity etc.)

On the other hand XLR often requires additional stage to "make" inverted signal. In electrically quiet place RCA can sound better IMHO (but I still like locking connectors). It is also application depended, as you stated. XLR has shield grounded on both ends creating possibility of ground loops in some systems. Also XLR is most likely more expensive - why to spend extra money if you cannot hear the difference?