I don't use GarageBand, but since it shares the same internal mic as iMovie I would assume the sound quality would be the same. Probably more audio tools in GarageBand. However, I am no expert in sound recording. The pro's have fancy microphones for that work.
I think that you'll need to experiment, perhaps with a small test movie, and see what sound quality is satisfactory to you. The voice over feature in iMovie would be the easiest to use. The voice quality is adequate, although I wouldn't say that it is super terrific. As with any audio that is not recorded in the same locality as the video it could sound a little like an add in. But it might sound natural enough for your purposes with the video you are working with. Perhaps using voice over with an external mic would yield better results.
In the old days before we had voice over, I used to record separate audio on my camera and then import the audio-video clip into iMovie and split out the audio. That seemed to give a natural voice, particularly if recorded in the same vicinity as the video was taken, and with the same camera. You would need to make the audio recording while running your movie in iMovie so that it matches up with the video when you import it in.
Hopefully others will chime in who have done some work with audio.
-- Rich
