I have the Progressive EMS portable unit, and it has saved us twice that I know of by shutting down when the voltage dropped below 108 volts.( 64 volts and 76 volts) Both times it was at night and I had both AC's running.
The other reason for getting the Progressive unit is that it has a life time warranty. Surge Guard only has 1 year warranty.
Mine got wet during a rain storm (not sure how, because I had it covered) and failed. One call to PI, and I shipped it to them, and had it back in about 2 weeks at no charge. ( I guess there was a charge for me to ship it to them, but it was minimal and well worth it.)