Originally Posted by humble_biker
Sorry to disagree but it is a combination of both. The dirty and less cared for chain obviously will ruin faster, but to believe that the forces acting on a chain doesn't cause the pins pulling against the plates to actually "bite" into them resulting in the chain appearing stretched is silly.
The average tension on the chain is no different with a Biopace chainring than it is with a round or elliptical ring of the same size.
If that's you main concern, go for big rings and sprockets. Better yet, make sure they are even numbered and keep the chain/sprocket phase relationship constant.
See:
http://sheldonbrown.com/chain-life.html for details on this.
Sheldon "Live Long And Prosper" Brown