Computer Musicians: Audio interface or USB mixer?

To put it simply, I’ve been looking to record external instruments into my computer (particularly guitars and bass) and I’d like to pick up some sort of audio interface.

I’ve previously been using Ableton Live and a headphone cable with ¼” adapter going directly through my Macbook’s soundcard, but the latency on that is so terrible it makes recording seriously impossible.
So I need to invest in some sort of interface. Would it be easier and more convenient to buy a USB/Firewire interface like this or to get a USB mixer instead?

I’d also like to be able to use the product I buy as a DJ system so I can run a headphone preview line out as well. (Essentially have two sound outputs).

So what product would be best for something like that?