You should choose a USB 3.0 or Thunderbolt harddrive to future proof your choice. Firewire is discontinued in RTF Lab Macintosh computers.
Thunderbolt drives are also a choice to consider but USB 3.0 is more accessible and backwards compatible with USB 2.0
G-Technology 1TB G-DRIVE mobile USB 3.0 Portable Hard Drive
G-Technology 3TB G-DRIVE USB 3.0 External Desktop Hard Drive