One of the most commonly used features on these things has to be the automatic low voltage protection. In the summer time with many running AC units, it's easy for the voltage to get dragged down below the AC's damage threshold level. Nice to know you can go out for the day and not come back to a damaged AC unit(s).
If you look closely at the specs and features between the Surgeguard and PI EMS, the PI has the edge. Plus with the PI, you not only get the lifetime warranty, you get to support domestic workers.
I just installed the 30A hardwired PI EMS.