I have both. I would say that I prefer the ESS. My first build was with the 57CNC for a G0704 conversion. While I liked the Pokeys initially I found that the way the board was configured was limiting. Most of the I/O has a fixed function. There are connectors for motors, one for a pendant, one for limits... When you try to do things that are out of the ordinary, like setup a console with lots of switches and lights, you will quickly run out of I/O. With the ESS it has just 3 connectors and the signals can be assigned to any function (within reason). The I/O on the ESS comes out to 3 standard 25 pin connectors. Also there are several BOB's out there that bring all the I/O to screw terminals instead of you having to use the ribbon cable connectors.
I have used the support facilities for each board and US based is easier and more responsive.
The BOB with the most for ESS.
https://www.cncroom.com/interface-cards/mb3
I have to admit however that I have only ever used Mach 4, and that I build my own custom interface cards to connect to the ESS and Pokeys.
Mill conversion with Pokeys
https://www.cnczone.com/forums/bench...are-posts.html
This is my lathe conversion with the ESS
https://www.cnczone.com/forums/verti...-software.html