Looking into building a new pendant and was wondering on what everyones thoughts are on both of these items and which one runs better with mach?

I've never ude the PoKeys device but I have used other keyboard emulators and I think the modbus is more reliable.

Pokeys is not a keyboard emulator, it can be used as such but can also be used as digital I/0 and is the way I would recommend if going for it. Never used ModIO so cant comment on it. I used the PoKeys when messing with my pendant and it worked well, even the analogue inputs but I ended up using my PLC as it allowed 24v which I prefer in an industrial environment.
Hood

Didn't know that.  I've never used ModIO either, but have also been using modbus through a PLC and it's worked flawlessly.  I thought PoKeys was a USB device.  Never had much luck with anything with a USB connection...Not with plasma anyway.

PoKeys is USB but it talks via a plugin. Never had any issues with it but that was way back in the early days so not tried it for a long time. Dont use Plasma so that may be something to keep in mind and like you say ModBus is rock solid so might be a better option, especially as normally 24v I/O is used with a PLC  although ModIO may just be 5v.
Hood