Currently have a Fireface 400 and am very pleased with it, however I am in dire need of more inputs and outputs. That made me look at the Fireface 800, but then a mate of mine asked me why I just did not get a Mackie Onyx mixer instead.
What would be the wisest choice and what is the main difference between a mixer and an Audio Interface except for the obvious looks?

The quality of the processing? A/D D/A Converters? Better preamps?
Also, if I get an Onyx mixer, will i need to buy an a/d d/a converter seperate or is one included? (cant find the info on the mixer site, nor see the mentioned Firewire port they say it has.)
Sorry for dragging the quality of these forums down with noob questions. Gotta start somewhere.
Thanks guys and girls!