Depends, dont you love that answer? In earlier times BI (before internet) thier was a clearer distinction now there capabilities overlap quite a bit.
If it is a stand alone application then I would go to with a PLC
If it will interface with a SCADA network then the next question is what hardware and protocol level will be used and who has that as a native format? Modbus, BSAP, Fisher ROC, CIP, Profibus/Net, etc. Almost everbody has add ones that will get you there but they add cost and lower reliability as add ons.
Then there is aftermarket costs to consider such as support availibility (does a tech need a airplane ticket or passport to get there?).
Also of concern is the number of IO points the Higher counts would go to a PLC.Now where that point is why you hire qualified intregrators.
Another concern is execution times PLC's are typically faster because communication is typically handled by a separate proccessor. So motion control and packaging will probably be a PLC.
But if I was acquiring a few points and sending them back to a scada system I would use something like a bullhorn rtu.