Man, the world of wheel building sure is overwhelming. So, double butted or straight spokes, which are sturdier?
In terms of the resulting wheel, typically double-butted.
Spokes work by distributing load to neighboring spokes. To do this well, they need to be under substantial tension; imagine trying to build a wheel where the spokes were loose pieces of yarn, only the piece currently on top would be taking up the load and it would be getting massively stressed.
Wider spokes are capable of being stronger, but there's a catch: they also need to be under higher raw tension in order to adequately distribute load. This is because thicker spokes are less elastic, so you need more tension to pull them to where it takes a lot of deflection before they'll go slack (therefore causing them to stop distributing load). So, if you build a wheel with wider spokes, you also need a beefier rim that can handle more highly tensioned spoke nipples. If you take a lightweight rim made for 14g spokes and decide you'll make a tougher wheel by building it up with thick 12g spokes, the end result can actually be weaker in addition to heavier; adequate tension can result in issues like rim cracking at the spoke nipples, inadequate tension can make it prone to failures like broken spokes.
Double-butted spokes are a clever solution: they're thick on the ends which is where failures usually occur, but they're thinner over their middle sections, making them more elastic and capable of distributing load better at a given tension. So they end up being lighter
and result in a stronger wheel than a straight-gauge spoke whose thickness is the same as the double-butted spoke's end sections.
The penalties of double-butted are:
1-They cost more.
2-It's a bit more difficult to build up and true wheels with double-butted spokes. They have reduced stiffness with respect to twisting, so they can accumulate twist more easily when the nipples are rotated. Mechanics have to be more careful to over-turn and then back off, so that the spokes are all passively untwisted.