Draft A gets the purpose right. It says a bloom filter is a probabilistic structure for testing set membership, and it names the property that matters most: there are no false negatives. Draft B ("a thing for data") is too vague to tell a reader anything and doesn't explain what a bloom filter is for. Both are one sentence long, so only the content separates them. Draft A could have added that false positives are possible and that it saves space, but it is still correct and useful.

WINNER: A   