I know this topic Is really old, but I want to share my personal expirience. I worked for a Rockwell's partner solutions company, so I had acces to a lot of customers and applications with Allen Bradlley PLCs, and now i'm developing in Mitsubishi Q series PLCs. You have to consider lot of stuff as JRoss explained, included the real needs for your customer and the added value you can offer as a programmer or a company. For small and specific applications, price will play an important role (small pokayokes, turn on and off a motor...) but nowadays technology is taking important steps and many other stuff is important to be aware.
For Example, traceability is one important thing on any aplication, communication and routing play a really important role in these days. For now, that im using a Q10 PLC series I discovered is really hard to develop a full solution, you have to pay for lot of more hardware and software, for example MES, windows server license, and othe software to be able to connect the OPC with the data base. Meanwhile Allen Bradley PLCs have a lot of compatibility with 3rd party software and open source products (as pycomm or pylogix, PTC OPCs, Ignition...).
For example, I had to develop a full solution for a small business with an Oven, total pricing with Allen Bradley hardware and software (Micro800 and Panel View 800 series) was under 10k dollars, becase I was able to buy a Ignition OPC or use an Open source software, meanwhile the cotization I received for Mitsubishi hardware based exceded 20k dollars, for the same solution, and we expected that our system with Allen bradley would be able to grow twice as its current size. i have to remark that this solution has covered routing (using Mikrotik routers), server, data bases, virtualization, web app, and control (where allen bradley PLC is used).
So, as you can see not everything is resumed to the PLC's hardware and software price, the full solution is what really matters when you have to choose or explain why one is or is not the best option.
Communication protocols should be considered as well, we are at the beggining of the IoT revolution, many protoccols as Modbus, Modbus RTU, and small modules must be considered, for example in my solution with micro850 I was able to find a modular remote IO (Novus brand) that included thermocuple Inputs and worked with Modbus TCP/IP, so I saved about 1k dollars to replace some Allen Bradley modules and save more than 2k if I had used Mitsubishi.