Florine has the highest electro negativity of any element.
Florine because of its high electro negativity never forms positive ions or acquires a positive electronically charge.
Metals form positive charges, non metals form negative charges.
While Florine never forms a positive charge chlorine does.
When Chorine forms polyatomic ions with Oxygen the chlorine takes on a positive charge.
Oxygen has a higher electro negativity than Chlorine so when Oxygen and Chlorines share electron density the electron cloud has a greater likelihood of being found nearer to the Oxygen. This leaves the Chlorine with a positive charge.
Because Florine has a higher electron negativity and never has a positive charge like Chlorine, Florine is more non metallic than Chlorine.