  A recent experiment [J. L. Garrett, D. A. T. Somers, and J. N. Munday, Phys.
Rev. Lett {\bf 120}, 040401 (2018)] measured for the first time the gradient of
the Casimir force between two gold spheres in vacuum at room temperature, and
placed a bound on the magnitude of the deviation of the measured force from the
proximity force approximation (PFA). The present work extends a previous
theoretical analysis of this experiment [G. Bimonte, Phys. Rev. D {\bf 97},
085011 (2018)], by analyzing in detail how the magnitude of the deviation from
PFA is affected by the inclusion or neglect of ohmic dissipation at zero
frequency, a much debated issue in the current Casimir literature, which goes
by the name of the Drude vs plasma controversy. We analyze as well the effect
of connecting the plates to charge reservoirs, which is the standard
configuration used in Casimir experiments. We describe a simple and effective
decimation procedure, allowing for a faster computation of the Casimir force
for large aspect ratios of the system.
