Differences between to_tsquery() and plainto_tsquery() when it comes to applying an index?

As a follow-up to my question from this past Saturday
(http://archives.postgresql.org/pgsql-general/2010-08/msg00624.php) I
experimented with adding two types of indices to the same text column:

CREATE INDEX item_eng_searchable_text_idx ON item USING
gin(to_tsvector('english', searchable_text));

and

CREATE INDEX item_searchable_text_idx ON item (searchable_text);

Running my queries under explain, I noticed that queries of the form:

select pk from item where searchable_text @@ plainto_tsquery('search
phrase');

Actually run *slower* with the item_eng_searchable_text_idx index applied.

But when I switch the query to use to_tsquery() instead, e.g. something
like this:

select pk from item where searchable_text @@ to_tsquery('phrase');

The performance is better.

Is this because the gin/to_tsvector() index works differently for
to_tsquery() compared to plainto_ts_query() ?

If so, how can I create an effective index for queries that will use
plainto_tsquery() ?

Note that I need the plainto_tsquery() form b/c my search phrases will
correspond to exact blocks of text, and therefore they will contain
spaces and punctuation, etc.