Copper has a lower weight when compared to lead. 
This means that copper bullets with the same displacement as a conventional jacketed lead bullet will be lighter and faster. We dont buy our shoes by weight; we buy what fits our feet and what we are going to do with them. So it should be with our bullets. Bullets need to match the twist rate of the barrel and the intended purpose. A bullets displacement and performance is of much greater importance than its weight.

Copper is considerably tougher than lead.
This means that even when fully expanded, copper bullets tend to shed much less weight than a conventional bullet and generally penetrate deeper and straighter as a result.

Velocity trumps BC.
Higher BC performance can easily be matched, if not completely overtaken, by lower BC bullets travelling faster. This is especially true over hunting distances of less than 300Ð500m. Use a Ballistic calculator and run the numbers to see for yourself. A further point to consider is that traditional bullets with high claimed BCs tend to have very small meplats (bullet tips) and often require higher velocities to expand reliably, reducing effective terminal performance range even further.