Hardwired!
In addition to the other advantages that were listed:
Why add another plug and socket? I often see where the plug or socket have burnt up on the portable units. If the receptacle on the portable gets a high resistance, you will get low voltage in your unit. I remove the wire from the input of the power panel, connect it to the EMS and then add a short cable from the EMS output to the input of the power panel.
And if it’s a 50 amp unit, the hardwired will protect your unit from a power cord fault. IMHO, an open neutral on the power cord (or source) is the most important protection.