What is the difference between aluminum and copper conductor?

So which is better?  Copper or aluminum windings? 
Believe it or not, transformers wound with aluminum or copper wire have similar losses and performance.  The choice of using copper or aluminum windings depends upon the application and the individual preferences of the person specifying the transformer.  Aluminum’s perceived problem dates to the 1960’s when connection problems caused several fires. There were never any problems with the actual aluminum wire. Modern, secure methods for connecting aluminum wire have been developed and are widely used. New methods of connections pierce the aluminum oxide layer in such a way that they remain gas tight to prevent further oxidation.
The type of winding material does not affect the transformer’s reliability.  A transformer’s life is defined by the life of its insulation system. Because aluminum-wound and copper-wound units run at equivalent operating temperatures, the insulation systems age at the same rate for each design.
An advantage of aluminum windings is weight.  Aluminum transformers weigh considerably less than copper transformers.  This is important in high-rise office buildings with serious square foot weight limits per floor.
Another advantage of aluminum wire is cost.  Transformers wound with aluminum wire have considerably less cost than their copper equivalent with very similar performance.
The bottom line:
Improvements in technology regarding the use of aluminum in transformers have made aluminum-wound transformers the ideal choice for today’s applications.  Here at MGM we can supply both windings with our transformers but most of the units we sell are aluminum-wound.  Customers get the same performance with less cost so aluminum is a more common choice.