Larry- From what I can see on Mark Williams' website, they are actually stronger CVD's, not universal joint based half shafts.
If this is indeed the case, they are worth the extra money. CVD's will not bind up the suspension under load like a half-shaft based system CAN in some circumstances. This will allow more compliance on the drag strip, but also on the road course if your car will see that duty.
The Untriax system is more of a band-aid fix in my opinion, by using adapters to allow a "gen-2 like" halfshaft system to be installed in a gen-3 rear. It WILL work, but it may cause more problems down the road. It was the way I was planning on going, but I didnt think I had a choice when I had made that decision...now I know different.
This falls under the KISS theory...Keep It Simple Stupid. What are you more likely to have problems with?
-2 CV joints and a shaft
-an adapter, 2 U-joints, 2-splined yolks, and a shaft
Easy answer...