I have both the Progressive Industries hard-wired 50 amp EMS, and a Hughes Autoformer.
They do completely different things:
* EMS -- Disconnects power from your coach in the event of many types of power anomalies.
* Autoformer -- Boosts low voltage up to a better range which helps your coach's devices operate more efficiently, and draw less amperage.
-- Devices will always attempt to draw the same amount of "power" (measured in watts) which is a product of ( Voltage * Current ).
-- When voltage goes lower, the current draw increases which leads to more heat and power loss due to heat, along with other inefficiencies.
-- The Autoformer boosts the Voltage up higher, so that the current draw can remain low (or closer to normal) while still powering your coaches devices.
I have mine installed in such a way that I can bypass it if needed ( only happened in St George -- the voltage was high to begin with, and the Autoformer always applies some small amount of boost ). I've also needed to move it from my coach to my car to charge the car and prevent it from tripping the breaker.
Here's a link to our blog post with more details.
Note that the Huges goes on the input from the city power, but I put the EMS on the output of the auto-transfer switch. It's unlikely your generator voltage would sag, thus no need for the Autoformer voltage boost there, but always nice to have more protection from all sources (thus the EMS).
** Edit -- I forgot to say.. I would definitely recommend BOTH. But if budget constraints dictate only one.. go for the EMS.
Hope this helps!
LK