Which is better Ni over tin plating or copper over tin plating on brass parts

Presently we are doing 3 microns Ni and then 8 Microns tin plating on brass parts. We would like to know is there any disadvantage if we do 3 microns Copper and then 8 microns tin plating on these brass parts?

Basically what is the purpose of doing Nickel or copper before tin plating on brass parts?

Dear Mr. Deasi,
Copper have the tendency to diffuse through the top layer and I am afraid the product may fail in your tests if copper plated.Black spots may appear on the plated surface sooner or later.Nickel plating acts as a barrier layer as brass you know is 80:20 Cu/ZN.As of now I think you cannot avoid or replace Nickel.Nickel plating also gives you a leveled deposit.Having said this there are two processes which can replace Nickel i.e; Palladium and White bronze with proper ratio of Copper/tin/Zinc.Generally 10 Microns of Nickel= 3 Microns of White bronze= 0.2 Microns of Palladium.The selection of the process usually depends on the requirements and cost.
Regards