I agree with Jazzman. As a general rule, a balanced (XLR) cable should be technically better than an unbalanced (RCA) cable, as it should do a better job of protecting the signal from electrical interference, especially over longer runs.
But then the choice of cable is also bound to make a difference – perhaps a good quality RCA cable is going to sound better than a lesser quality XLR. Or maybe in your setup it wouldn’t make too much of a difference and you could end up paying extra for a balanced cable and not hear any significant improvement. So, as Darko suggests, it’s probably worth listening to the difference for yourself if that’s an option.
Personally, I would always choose balanced/XLR, but that’s my bias.