Detecting metal, detecting a coil - which is easier?
Your question is not specific enough: what is the shape of aluminum piece? Are the copper turns flat (in plane)? what is the orientation of the copper turns with respect to detection coil? And so on.

And why do you compare aluminum piece (2.7 g/cm3) with copper (8.9 g/cm3)? It would be more logical to compare the same material.

Under some reasonable assumptions the answer is: solid piece of highly conductive non magnetic material makes smaller signal (on inductive detectors) than wire of the same volume, assuming the wire orientation is optimal for detection.