External Sound Cards for DJ Cueing

[dankodyssey]

I'm new to Ableton, I just got an APC20 with Ableton Lite to get started DJing and such. I'm running a Macbook and need an external sound card so I can listen to cued tracks. What are the best options out there? I'm thinking I should use a USB or Firewire connection.

[CivicRydr]

Budget: Focusrite Saffire 6 USB
Baller: Apogee Duet2

I have both of these and they work great. Both have a dedicated headphone output with selectable source.