I have seen such presentations which stipulate presentation parameters (much more strictly than TED's 18 minute limit) and always wonder for whom these limitations are desired, and what's to be gained? Especially in an academic setting, as you say.
You wrote: "In other words, is PowerPoint more cross-platform friendly and perhaps even a bit easier to use?"
Answers: in terms of the presentation setup as described, PPT is the more cross-platform friendly, at least until iWork in the Cloud is adopted in your setting as a viable platform. But if you're permitted to bring a Keynote file, then any advantage of PPT is rendered moot. It is definitely not easier to use, especially if you will be employing video.
Will you be advancing your slides manually each 20 second or is it to advance automatically or by a third party with an eye on the clock? In which case - and I may be wrong - PPT may give you more flexibility in terms of slide timing automation.
Do remember, for both KN and PPT, that if you deliver your file in a USB drive and you are using some esoteric fonts, the local laptop may not contain fonts, and will instead default to Helvetica or Arial or similar genericised font family. I have seen more than a few academic presentations fall over this way, and it also affects layout as well.
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