Waisted type will distribute the tensile load more evenly along the length of the spoke. Technically more purist and saves a little weight, but quite a small margin in my view.
Effectively, waisting the centre section allows for a better match in pure tensile performance to both the reductions at the thread and bent section of the design.
Straight spoke "should" snap either at threads or bend (often it's the bend) as the increase in load capacity of the centre section places more concentration of load at those sites.
As I first noted, with a production wheel, the design and sizing will usually have some decent margin in it for longevity, this being sensible in avoidance of failure in reasonable terms. which makes it only a marginal choice in reality, either should be ok.
