Scanning Hardware

We are in the process of acquiring several scanners for use in a document management system. We have regular 8.5 x 11 documents as well as driver licenses and other odd sized paper documents to scan. There will be times that we will scan 50+ documents at one time.
Our choices are to get a roller-fed scanner or a gravity-fed scanner. Does anyone have an opinion about which would be better overall for our purposes? I understand that a gravity-fed scanner would probably be fine for single documents, but I am concerned that multiple pages would not feed well, at least that's been my experience with gravity-fed printers.
We will have to purchase 9 of the scanners and we'd rather not make a mistake.
We are currently considering the Canon DR-M140 or the Canon DR-M160.