Why choose an audio interface over a mixer?

Question

I know there are tons of question out there who are quite similar to mine, but yet none of 'em seem to be able to solve my question.
The main question I'm asking is: what's the difference between a mixer (either w/ or w/o built in audio interface) and an external audio interface?
But the answers to this exact question weren't sufficient because I ran into a practical problem, wich is as follows:
A friend of mine uses his Behringer XenyX 802 mixer as a way to input the signals his mic's send into GarageBand/Logic (kind of DAW is irrelevant). Now i've been wanting to set up my own little home studio myself and basically every tutorial/video out there says: "in order to record multiple instruments into your Mac, you need an audio interface." Now I don't get exactly why, because i've seen someone do it with a mixer. I assume that the mixer in question has a built in audio interface (since it has, amongst other features, phantom power), so it's not neccesary to have an audio interface, or is it?
Then I started searching the web for different audio-interfaces because I thought: "well they must know what they're talking about. So I guess I have to start looking for an interface." I found that a lot of interfaces <€100 have only 2 inputs, mostly an XLR input and a Jack/instrument input. One of the most popular beign the Avid Fast Track MK II USB interface. I happen to know that the Xenyx 802 has 2 XLR inputs AND 2 jack inputs and I can get my hands on one (2nd handed version) for about half the price of the Fast Track. I thought to myself: "why? Why do all those 'experts' who explain it in video's tell me to specifically get an interface?" this is because I found that the more input channels you want from an interface, the higher the price.
Can anyone help me explain why the Fast Trak MK II is twice as expensive as the Xenyx 802 while it offers half the amount of inputs?
Thanks

Answer 1

Cyclone13 wrote:
Thank you for your response, it helped quite a lot. Just to make sure if I understand correctly:
You can only record 2 instruments simultaneously at max using a mixer (on their own track, because I have seen it done before), but you would be able to record, say, 6 mic's at the same time if you had the proper interface (an interface with at least 6 inputs and outputs that is). Now the M-Audio has only two inputs I believe, and therefor I take it has a stereo output as well, so the maximum simultaneous recording for the fast track is 2, just as that of the Behringer?
Regarding the Mixer... The Behringer you posted is just a standard Mixer, it doesn't have USB output, in which case you have to use the Mac's audio system (the Line-Input) and run a cable from the Mixer's main or monitor outs into the Mac's Line input. Computers use inexpensive audio chips that do not provide the greatest sound.
I thought you were talking about this Behringer:
http://www.amazon.com/Behringer-Xenyx-Q802USB-Audio-Mixer/dp/B008O517IC
It is a mixer and an audio interface rolled intro one.
The M-Audio is a true separate Audio Interface that provides it's own Inputs and Outputs, (Like the USB Behringer).
Regardless, all three, the Behringer stand alone mixer that uses the mac's audio, The Behringer with the built-in stereo USB audio interface and the M-Audio with USB interface are all stereo units. The USB audio interface will only show two inputs and a stereo out. The most you can record separately on either is 2 instruments.
Here is one step up:
http://www.motu.com/products/motuaudio/audio-express
The MOTU unit has 4 analog inputs as well as a stereo digital input.
The next step is an interface like this with 8 Mic inputs. It would show up in Logic as 4 stereo pairs or 8 individual inputs. You could use 4 mics on a set of drums, 1 for guitar, bass direct, stereo input for keys and record everyone playing at the same time on 8 separate audio tracks giving you full control (in Logic) of the final mix.        http://www.motu.com/products/motuaudio/8pre