Get one where the (number of) inputs and outputs suit your purposes. I wouldn't go external solely for improved signal-to-noise-ratio, that's not an issue for quality internal cards either. An external audio interface's strengths are in portability, easier jack access and volume control at an arm's reach. The latter is of course true with an internal card paired with a set of active monitor speakers.
Departing from basic multichannel integrated audio and going prosumer and up, rather than looking at DACs, numbers and technical specifications (other than an optimally flat frequency response) and trusting they're good enough at that level for sub-golden ears, I would emphasize the OS driver quality and maturity. Get the user experience testimonials on whether the card performs reliably or is it unstable, which could mean audio glitches, problems propagating to the OS (errors, crashes) or that it simply shuts up and makes the user jump through hoops to get some sound out of it again.
