Bill,
Pokeys is a great wee device but I would say the PLC is the better option for a tool changer especially as you already have it. Main reason is your tool changer is very likely to be 24v and the PLC can interface directly to that (presuming its a DC or relay out PLC) , with the pokeys you would need to convert the 5v to 24v and vice versa.
The Pokeys is very nice for switch panels and hand held pendants and is a nice price when comparing to a PLC, especially if you want analogue Inputs.
Hood