After sales support is perhaps one of the most important issues to consider after operational needs have been met. Also bear in mind the fact that you will, if you get one of the larger systems, probably need someone to manage the system once you have it through the door.
Before selecting spend a lot of time speaking with operators who have the system already and who match your type of operation, ensure you IT bods are part of the selection process.
The above posts give quite a balanced view on most systems. My expereince is that none of them are perfect but you can make a system fit your type of operation.