X-rings are an improvement to the O-ring. They both seal the internal lube in and keep the dirt out. X-rings have less resistance/drag and seal better and last longer.
Allow me to add one more dimension to this thread if I may . . .
Moose Racing now offers a 520 "FB" chain - anybody tried one yet? They now offer the FB (Fine Blanking) instead of the X-ring yet still offer the O-ring and non o-ring chains. Link attached.
Moose Racing is proud to reconfigure our chain lineup for 2007. We are introducing the FB, and leaving the X-ring chain behind. The FB is a newly developed top of the line chain. This is the highest quality chain we have ever offered, and we are bringing it to you at a lower price than our X-ring without decreasing quality! After more than a year of testing we are importing what we believe to be the last chain you will ever switch to, the FB.