The benefit of an albeit modestly more expensive model such as the HP OfficeJet 8600 (It comes in 3 "flavors", but operationally, they are pretty much the same), the key advantages are cost per page of print and durability.
Price per page of print on the 8600 series beats that of most other ink jet printers as well as almost all laser printers. The 4600 is a decent printer, but ink costs will be somewhat higher. The 6520 series falls in the middle.
