What are the pros and cons of Double Butted vs Straight Spokes when building a wheel? Straight spokes will have the same diameter through the whole length, double butted spoked are thinner in the middle than at the ends. Here's my current understanding of the benefits of each:
Double Butted Spokes

Lighter than Straight Spokes
More elasticity in the wheel (less rigid / will flex some before breaking)
Spokes will bow out less at the crossings
Easier to bend/lace
When overtightened or stressed too hard, the spoke is more likely to snap than the rim eyelets or the hub's flange

Straight Spokes

Stronger than butted spokes
Less elasticity in the wheel (more rigid / may break before flexing)
More available at bike shops
Cheaper

What are other reasons to use one over the other, or more pros/cons that may influence one's decision to use one over the other when hand building wheels? ('Straight spokes are easier to machine lace' may be true, for example, but doesn't help me decide which to order)
In his book 'The bicycle wheel' Jobst Brandt, says that double butted spokes will be more resistant to fatigue failure when built into a wheel. 

This is because spokes break because of the cyclic stress they suffer as the wheel rotates. 

As the spoke rotates thru the bottom of the wheel it experiences a reduction in tension. 

Butted spokes are more elastic so this reduction in tension is spread over more spokes, each experiencing a smaller reduction in tension.

Wheels are an order of magnitude stiffer than the tyres they roll on, so any stiffness differences will not be noticeable. A wheel built with butted spokes will be able to carry a heavier load before any of the spokes become slack. At this point the rim is no longer restrained and the wheel will be more likely to collapse.