The biggest cause of chain stretch is running the chain too tight. The number of singlespeeds I see on here with perfectly straight chains from cog to ring; blimus!
Run looser, like on my Solitaire, they last forever...
But under load (which is surely when the wear occurs) the top run of the chain is as tight as, well you know, a very tight thing (so actually under tension, then) and the bottom run will then be even looser than it appears to be when the bike is "at rest", so to speak.
My view on this is to run the chain as slack-free as possible but without there being any noticeable drag - so that if you spin the cranks backwards they'll spin for a couple of revolutions.
I would suspect that the reason that you are of the opinion that loose chains last longer is because any incremental wear is less easily spotted than if the chain was run at the sort of tension that I use. Otherwise, unless we're talking about stupid amounts of actual tension (not just taking up slack), I can't see why it would make any difference as the tension in the top run of the chain will be the same for a given loading.