postgresql or xquery?

Readers,

Admittedly, a biased place to ask (!), but for a new database project,
how best to evaluate whether postgresql or xquery should be used.

As a novice of postgresql and xml, am not sure what is most
appropriate for a new database that will be tested initially on a
local computer, then transferred to a web server at some point in the
future.

For now, just reading about the two technologies before deciding. It
seems that an established system would be: data entered to various sql
tables and stored on server; php used to query table and deliver data,
for example as xhtml and css; user views query results as a table of
values in a web page. What does xquery do as a better advantage (if at
all)?