I recommend using the ZoomH6, namely because if you're going to be spending money on podcast equipment, you might as well kill two birds with one stone and get an interface that is also a digital recorder, so that you can record directly to it when you're on the road or away from your computer.
In order to have each microphone record to its own track in Garageband, you'll need an interface. Simply plugging two mics into your computer will NOT work. I recommend using the ZoomH6, namely because if you're going to be spending money on podcast equipment, you might as well kill two birds with one stone and get an interface that is also a digital recorder, so that you can record directly to it when you're on the road or away from your computer.