I know this discussion has been hashed out a bunch of times. I've read many threads, and i'm still confused. Not so much by the content, but by the markedly different opinions.
As i see it, many ppl argue that wheels built with DoubleButted spokes are stronger because you can get more tension with the DB spokes, which compensates for more lateral (side-to-side) stress. Also, since there's more "give" in the center of the DB spokes, it relieves some stress on that part of the spoke. And, if you're comparing 2.0/1.8/2.0 DB spokes, the width (and, presumably, the strength) at the j-bend and the threads will be equal... these are the spots where spokes tend to break.
The folks who champion (pun only partially intended) PlainGuage spokes say that while DB spokes are stronger spokes, PG spokes build stronger wheels. Less flex, they argue, gives the rim less chance of dealing with detensioned spokes, and less chance to deform radially or laterally.
Most ppl i know who build wheels prefer doublebutted, but are hard pressed to explain why, beyond "higher tension". Researching online has yielded conflicting information. The question i'm posing is: what creates a stronger wheelset for an overweight guy or someone hauling stuff on racks: DB or PG spokes?