Which is faster: BETWEEN or 2 WHERE statements?

Psql 8.4

Which will help the query run faster on my version of Psql;

1) WHERE (mydate >= '2015-01-01') AND (mydate<='2015-01-31')

2) WHERE (mydate BETWEEN '2015-01-01' AND '2015-01-31')

I'm just curious.  We have 500,000+ records to go through on this one table, and query involves 2 tables. The date field 'mydate' is not indexed.

Thank you!