Do you need bandwidth or IO performance? Bandwidth buy Seagate, IOs buy Hitachi.
Regular or enterprise? If you consider your usage to be a high duty environment or you like to keep drives forever or you won't notice the price difference, buy enterprise, otherwise buy regular.
OEM or Retail? Whichever is cheaper. The drive either works (yay) or it doesn't (RMA) and the manufacturer isn't going to provide any useful support to you.