Aluminum voice coil is lighter.Take in point the JBL E-140 with copper voice coil is 100db/watt and the E-130 is 105db/watt.
The E-140 is a deeper voice coil than the E-130 but the E-140 is copper and the E-130 is aluminum used for lighter weight to wrench every last bit of efficiency from the driver.
Reason I know this is because I have a pair of E-140's being reconed to E-130's and this discussion came up on lansing heritage forum.
Ron