None. The signal is measured in voltage, so the two terms are
synonymous.

Then why did you ad the parenthetical "in terms of voltage"?

I put it there before I added the "into the same speaker" bit. I forgot to
delete it. It turned out to be a redundancy. If you're not going into the
same speaker, then you need to measure both voltage and current to make
certain that the amplifier output is indeed equivalent in both cases. It's
a control.