Reply to USB Mixer vs Audio interface

Hi guys, Ive reinstalled Windows-7 (64bit) on my PC and suddenly my internal Maya soundcard (PCI) doesn't work anymore. It's been giving problems even before the instal but I believe its properly busted now. I've taken it out and tried it in a different PC but I need to wiggle it in the slot before it gets power and then it just switches off randomly. Not sure if it could be repaired as Ive cleaned the contacts with no success.

So now Im in the market for either an entry level external Audio Interface (Alesis i02 OR SIMILAR) or an entry level mixer with built-in sound card (Behringer XENYX Q502)....both USB powered. Both units have similar functions but I need to know what is the difference between a USB mixer and a USB Audio Interface.

I basically want it to get better sound quality to my active Monitor speakers (Alesis M1 520) when running Cubase or Reason and to connect an electric guitar and mic to record into Cubase. The mic is more to record sounds (and tweak them) and not really to record vocals at all and I wouldn't ever need to record more than 1 input/instrument at a time. I would also be running Traktor via the Interface or Mixer, to the monitor speakers instead of using my controllers soundcard.

So I have no idea which 1 to get. At this price range I believe sound quailty would be similar for the 2 units and not great, but hopefully better than my PC standered onboard sound.