My stringer tells me that gut is a better deal because it holds its tension and it lasts a lot longer. This seemed counterintuitive to me, but he says it's quite obvious to anyone who strings it a lot. One problem with natural gut is that you've got to really protect it from moisture.
I'm not a string breaker, so I've really enjoyed the Tonic gut in my ncode 95. If I was one of these huge hitters who breaks a string every couple of sets, then it would not be affordable. Then again, neither would Excel or NXT.