Quoting from online Oracle 9i SQL Reference:
“Specify DISTINCT or UNIQUE if you want Oracle to return only one copy of each set of duplicate rows selected (these two keywords are synonymous). Duplicate rows are those with matching values for each expression in the select list.”
So, there should be no difference in the two (performance or otherwise) as they are treated the same by Oracle.
On another note, don’t try to use UNIQUE in SELECT statements for other databases (MySQL, PostgreSQL, etc). You get up with an error because ANSI standards use DISTINCT with SELECT statements. Using UNIQUE with SELECT statements is just a product specific remnant of older Oracle DBs created before the ANSI standard was born.