I take it by your post that you're perhaps thinking about components that have XLR connections or just want to find out a bit more about them.
As far as a connector goes, the XLR is superior to any RCA connector out there. The XLR is more robustly built, it latches, and makes for a very secure connection.
The XLR input/output is a balanced connection and this makes for reduced noise pickup in interconnects between components. Now if noise is being transmitted by one of your components, say the preamp, then XLR will do nothing to eliminate or minimize that noise. For that matter, neither will an RCA. One needs to pay particular attention though when a manufacturer especially in consumer audio puts an XLR connection on their unit. Some do not employ fully balanced differential circuitry which tends to negate some of the benefits of the XLR.
Now even those people who use RCA's still have a way of achieving improved immunity to noise pickup. One can purchase isolation transformer devices that sit between the two components. These are available from a few sources such as Jensen Transformers and a product called the EbTech Hum Eliminator. For people who've got a particularly nasty EMI that's making it's way into their cables, these approaches can be very effective.
You may find the following two links of interest as they delve a bit more into balanced connectors and touch upon things like ground loops.
http://www.rane.com/note110.html
http://www.rane.com/note151.html
As in so many things in life, audio can be about adopting an approach, a design philosophy if you will. For people looking to monoblock their entire setup and keep the amps right next to the speakers, they'll have to run long lengths of interconnects. In situations such as that, it makes sense to give some serious thoughts to XLR. Some people want to take a prophylactic view towards matters and they search products that have inherant benefits when it comes to dealing with picking up extraneous noise.
If you see yourself running short interconnects, then it's really a 6 of one 1/2 dozen of the other as to which way you go. If you're running or going to be running long lengths, then more serious consideration should be given towards XLR.
Pricewise, you might find yourself pleasantly surprised at how inexpensive the cabling can be. Of course, if you buy some of the audiophile labled stuff, you may be in for a bit of sticker shock!
