I do both.
Story design. Contrary to good practice I usually start bottom up, designing one or two slides to get the look and feel of the presentation master right, then usually the "killer slide" that captures the most important message of the presentation. After that, I go top down and layout the entire presentation.
Slide design. Contrary to most people I go top down, loosely putting slide elements and scribbling text that is imprecise and not final yet, cleaning things up later.
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