Sonnet writes better research drafts, Haiku writes them for a cent
TLDR
The test. Haiku 5.5 and Sonnet 5.5 each wrote nine research drafts from identical source packs, using one fixed template. Two AI judges compared them blind, 27 verdicts in all.
The result. Sonnet won 16 verdicts and Haiku 11. Sonnet is better at insight, accuracy and readability. Coverage is tied. Neither model invented a quote.
The twist. It depends on the sources. Sonnet won 15 of 18 verdicts on messy news-style packs by cross-checking and catching errors. Haiku won 8 of 9 on a dense documentation pack, where breadth is the job.
The price. About one US cent a draft for Haiku and nine cents for Sonnet.
My take. Haiku for first-pass drafts on well-organised sources. Sonnet when the sources disagree or might be wrong.