Depends what you want to do. The default query for the search web part
primarily uses freetext. This will give you a broader result set.
Freetext will use stemming and thesaurus. If you search for run, it will
look for run, ran running ect and any words in the thesaurus that
matches these. If you use a phrase as a search term freetext will look
for each individual word as well as any combos of the words in the
phrase. I believe where freetext looks (in all col) depends on where
you tell it to look, it does not have to be all cols. Contains is a more
exacting It will not stem or use thesaurus. It will not look for
combinations of the words in phrases.
Generally use freetext if you want to get a larger (less specific )
resultset. Use contains if you want to exactly match.