the BUS which they want to used depends on the existing environmental, speed and cost.
dose your customer really need very low latency time?
I think you could discuss these with your customer.
Yes of course there are a lot of options that can be proposed and discussion with the customer is forthcoming. The customer is not particularly electronics savvy (they are more mechanically oriented).
What I'd like is some background on ethercat from someone whose used it to get the real scoop on it's robustness, ease of implementation and supportability. They guys who sell fieldbus implementations like ethercat are great at selling how easy it is to use and implement. Then when your past the point of no return you find all the ugly hairy bugs, problems and complexities that the vendor didn't tell you about.
Since this is used for PID, yes you could conceivably need low latency depending on the process being controlled and it's bandwidth. For this application a latency of a few ms would probably be fine though I'd be more concerned with inconsistency of jitter and determinism of that latency.