IMO a regulator can do much more for you than a simple isolator, but the isolator can eliminate DC on the line, which is kind of rare, increase filtering due to increased inductance and make the incoming AC immune to certain types of EMI/RFI noise pick up.
In various places I've lived though the automatic voltage regulator is what really kept my gear fed consistently well, whether from too low or too high a voltage.
Linear amps in particular may both cause voltage sags and be subject to them. A regulator will make the incoming AC look much stiffer.