This is a metal detector question, with one assumption. That is, the detecting is done by eddy currents in the target changing the resonant frequency of the detecting coil.
The question then becomes, would it be easier to detect (say) a piece of Aluminum or a few turns of copper wire (closed circuit coil) of the same mass?
That is, the detecting is done by eddy currents in the target changing the resonant frequency of the detecting coil.
The question doesn't become what you think it does because many metals will have resistive losses easily detectable with hardly any discernible change in the frequency of oscillation of a tuned search coil.
The resonant frequency will also change (with virtually zero eddy currents) when a piece of ferrite is brought into the search head field.
At hundreds of kHz, stainless steel produces both ferromagnetic and eddy current disturbances in the search head field and, these two effects drive the oscillation frequency in opposite directions hence, there can be zero net change in the frequency yet quite sizable resistive losses detectable.
However, if you compared a flat sheet with a multiturn shorted coil the coil will usually win because the flat sheet has the inductance of 1 shorted turn yet (say) 5 turns in the coil will have a leakage inductance 25 times bigger. In other words it will have a bigger disturbing effect on the field.
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.
