There is more to it than just a joule rating.
The PI EMS is rated for a peak surge current of 44,000 amps. The Surge Guard is rated for only 6,500 amps. The Hughes autoformer doesn't list a peak surge current and only the joule rating (2400). The response time of the MOVs is also a factor.
In a "whole house" application as a comparison, it is recommended that surge suppressors have a rating of at least 40,000 amps.
The Hughes autoformer and the portable surge suppressors mentioned by the OP are not UL listed (but their hardwired versions are).
If looking at strictly surge suppression, I would opt for the PI portable unit. But why not get an EMS unit that protects against other potential power problems like low voltage, open ground, open neutral, etc.
