It depends.
If your docs were orders where you wanted to know "which products are typically also bought with pasta?" then you might use a keyword field and significant_terms because you'd be examining significant patterns in repeated orders for exactly the same product.
If your docs were products you'd (hopefully) only ever have exactly one unique product description so the keyword field would be of no use with any significance analysis (everything occurs once). If you were looking at some of the ingredients in the text of these descriptions (eg. common ingredients mentioned in high-fat products) then you might use an analyzed text field and significant_text. Maybe indexing with shingles would help too. Remember the indexed field you search on can be different (eg stemmed) from the indexed field you use for significant_text analysis (e.g. whitespace)