Should I choose interface or mixers?

Do you want to mix as you record, or record separate tracks and mix it later?

Have you looked at "standalone flash recorders", such as the ZOOM H2?

It has 4 built-in mics that can record "in surround" if needed.

Once recorded, the sound files are easily moved to the Mac for archiving or editing.

Both.

If you're not doing any editing and you're just recording your band jamming, you will need a mixer for mic pre-amps, levels, a bit of eq etc.

And.. an Audio Interface to capture whats coming out of your mixer and convert it to digital audio, in your macbook.

Your 'mic in' in your macbook is simply not good enough to record audio.

edit: The above recommendation of a flash recorder is a great and much cheaper option.

you don't need both. it's down to how you want to record.

if you want to do it like you're playing a live show, then:
1. set up a bunch of mics
2. run them to the mixer
3. mix it to stereo
4. feed the stereo mix to the computer (perhaps to a dedicated interface, though not necessarily)
5. record the stereo mix on your computer

if you want to do it like it's in a recording studio, then:
1. set up a bunch of mics
2. run them to a multi-channel interface
3. record them as separate tracks into the computer
4. later, edit/mix at your leisure

if you want to do it like you're attending a live show, then:
1. get a Zoom recorder, as suggested
2. set it up in the room
3. record to it
4. dump the tracks into your computer
5. later, edit the stereo recording at your leisure

the 2nd setup gives you the most flexibility. the 3rd will sound the worst, but is the cheapest. the 1st will require the most work up front, the 2nd will require the most work in post.

I highly recommend the Zoom H2 as well....if its just to get the songs down and no editing you just can't go wrong. Its quick and easy, no hassle and the sound quality is amazing. You have to hear it to believe it. Prob costs abo 300. I imagine the Zoom H4 is even better but I haven't tried that one yet.

Put it this way, you pull it out of its case....flick the switch, press the record button twice and you're recording. No cables, no troubleshooting, no mixing.

Drop it in the room like they used to record the old school jazz records.