Acetate ion has charge delocalisation and in Br- charge is distributed throughout the atom ,so how should they be compared?
Br anion is much larger in size than oxygen anion present in acetate ion. Hence Br anion can hold the negative charge better than oxygen atoms of acetate. Again negative charge density in Br anion is less than that on the 2 oxygen atoms of acetate. Therefore , Br anion is a weaker base than acetate and it is very well known fact that weaker base is a better leaving group or nucleofuse.