Well depending on what mixer you have, the preamps on it may or may not be better than those on board your particular interface, so you shouldn't feel you necessarily have to use it (unless you're running multiple channels into the mixer and outputting the stereo mixdown to the interface, so you have to use the mixer preamps
) - but even putting quality of preamps aside, the other differences between interfaces I would say are 1) quality of A/D and especially D/A converters; differences between the latter in different models are much more noticeable than the former, because the whole mix is being channeled through the D/A converters, to the stereo output, to your monitors/speakers/headphones, so inferior D/A converters will have a less defined stereo image and seemingly not as full low end (and these statements are from personal experience) 2) Driver stability, this I'm finding is a big one - how easily and dependably the computer recognizes the interface when it's connected; how often it drops out or has conflicts with other devices (such as internal wi-fi cards) that have to be disabled; how low you can get the latency while maintaining stability; and if you're on a PC, how well WDM playback is integrated (meaning, for all sounds outside of the DAW, e.g. music playback, gaming, youtube, etc.), to name a few, and of course, 3) feature set - how many outputs it has, whether it has ADAT capability or not, how many headphone outs, dedicated monitor control knob, etc.
