Which one is more accurate for giving stable power over line voltage fluctuation?

I need to run a compressor of 500watts with a 200watts condenser blower fan. Here the line input standard is 220 volts but sometimes the line voltage remains 190volts other times it is 265volts. So to save the chilling unit from heavy fluctuations, do I need an automatic voltage stabilizer or an automatic step regulator? The shop said that step regulators will cost less and save money. I just want to know which one should be better for more precise power output? 10points to the best answer.