Alloy wheels new scorpio?

i have bought a new suv scorpio so i wanna know about wheels plz give ur knoledge my car has a 16 inch rims (wheels) i wanna change with alloy wheels so i wanna know that which size of alloy wheels r better 15'' or 16'' inch my car model scorpio 2.2 vlx which tyre size is better plz tell any effects on performnance