As long as you are comparing Garage Band to DAT, GB will be a better tool, simply because it gives you better editing capabilities (like crossfades). The quality will depend on what kind of audio inputs you are using. The S/N ratio of a DAT recorder typically is better than the internal minijack input of a Mac, but many external audio interfaces beat the DAT by a long shot, quality-wise.
If you are to do multitrack recordings, however, Garage Band (or what I know of it) will not let you do this. It is not uncommon to record conterts using one or more stereo pairs PLUS several ambient mics. Some audio engineers even prefer close-micing instrument groups in addition to a stereo pair ambient mic, no matter what kind of concert we are talking about (even classical). If you are to record such an event, a pro app like Logic or Pro Tools is essential.
