I don't work at MIRI but: in the movie the guy cranks out a novel in like one night. That's years of work compressed into a few hours. He then proceeds to understand enough about markets to become extremely wealthy (thus negating the time travel informed betting angle), itself a many-year task. Most importantly: NZT-48 lets him figure out how to make MORE NZT-48 with fewer side effects, thus ensuring an indefinite supply. The NZT-48 is definitely the correct choice.
Approaching the question from the other direction: How much has MIRI really accomplished in the last 5 years? I think it's safe to say that a large part of what they've achieved is in terms of popularization and awareness raising rather than actual research/information generated. If they sent back a flash-drive to five years ago, that wouldn't jumpstart them anywhere near 5 years in terms of effort invested.
NZT-48 lets him figure out how to make MORE NZT-48
Presumably sending one flash drive back can be accomplished every 5 years in a classic time loop, letting MIRI figure out how to send MORE and more advanced data back, eventually designing the FAI in 5 years total, without having to worry about the competition.
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