What a difference a diamond makes. Casting its eyes jealously up and down the Strip for years, the Venetian has longed to join the highest tier of luxury hotels by winning top honors from AAA and the Mobil Travel Guide. The competition grew more serious a year ago after the two-year-old Wynn Las Vegas next door won its first AAA Five Diamond. Wynn and the Venetian are engaged in a rivalry for the same customers, from high rollers to spa aficionados.