the gauge can make a difference when you have long runs.
currently I'm using 12 awg speaker wire made by Liberty Cable. The first thing I noticed about it was how fine the individual strands were and also the number of strands. It simply looked better than any other speaker wire I had seen. It was definitely much nicer than other wire I've seen for the same price, $1 per foot.
Now, I have never done a direct comparison with Monster XP. So far as I know, XP is exellent wire. I have used some of the base grade Monster speaker wire and I didn't care much for it.
Ultimately, it comes down to whether or not you are pleased with the sound.