We support several HP Scanjet models. Each model has the same resolution (2400 X 2400), and operates on a USB port. The differences include such features as the size of the flatbed and the scanning speed.
For most purposes, the HP Scanjet 5550c (for scanning multiple documents) or the HP Scanjet 5500c (for scanning photos and slides) would be fine. The HP Scanjet 7400c would only be needed if you wish to scan legal-size documents.