There is a big difference between the two products. The SSP-50 surge protector does simply that. It protects against power surges that can be caused by power fluctuations, lightning strikes, and heavy electrical loads being switched on and off. The SSP-50 will also indicate with LEDs if a campground outlet is incorrectly wired. The LEDs allow you to test a campground outlet for certain faults before attaching your motorhome power cord.
The more expensive EMS-PT50C does much more. When first connected to the campground outlet, it performs a series of tests for incorrect wiring, high and low voltage, and frequency. If all the tests pass, then a big relay inside the device will allow power through to your motorhome. The power is constantly monitored, and if any faults are detected, the power is automatically shut off. The EMS-PT50C also has surge protection like the SSP-50.
Most of us would recommend the more expensive unit. Low voltage is very common at older campgrounds here in the USA. This can cause damage to your air conditioning compressor as well as other electronics inside your motorhome. On a hot day when many campers are using their air conditioning, the voltage can easily drop due to the electrical demand. The PT-50 will disconnect the power if it drops below 104 volts or is above 132 volts. Power is automatically restored when the voltage is safe again. High voltage is less likely, but can occur if there is an accident.
I recommend the internally installed version EMS-LCHW50 or the similar version with a remote display EMS-HW50. Because these models are installed inside the compartment where your power cord is located, they are protected from weather and theft.
I have an EMS-LCHW50 installed in my Georgetown, and there have been several times when it disconnected the power to protect my air conditioner and other electronics because of low voltage. It has also prevented power going into my motorhome at a campground that had no ground connection, which could be a shock hazard. One time it detected and blocked power when there was a disconnected neutral, which is very dangerous. In the USA our power is 120 volts. A 50 amp campground outlet has two 120 volt connections with a single neutral return. If the neutral was disconnected, there is now 240 volts of power, and all the electrical devices in the motorhome would be damaged or destroyed.
Progressive Industries makes excellent products, and has great customer service. Many RV stores sell their products, so shop around on the internet to find the best price.