english210 - This is a lot of discussion, considering that there is usually not much price difference between the RCA & XLR versions of the same cables.
1. If your amp isn't a balanced circuit, it makes no difference.
2. With balanced circuits, you may get a slightly better signal-to-noise ratio with XLR... with the emphasis on SLIGHTLY. This can be a much bigger difference with very long runs of cable.
3.The Cable Company's lending library is an excellent suggestion. As always, best to hear (anything) in your own system before buying.
My suggestion for a bargain XLR (or RCA) interconnect is UIT (about $800/pr). Just discovered them and I'm very impressed, esp. at that price-point.
I hope that helps.