GarageBand '09 and Audacity Noted Differences

GarageBand '09

Audio Quality
 - 24 bit / 44100 Hz

Specific Effects:
 - Noise Removal: NO
 - Tempo Change: NO
 - Change Speed: NO
 - Click Removal: NO

Real-Time Effects: YES

MIDI Recording: YES

Software Instruments: YES

Built-in Loops: YES

Overall Editing of Audio File: NO

Audacity

Audio Quality
 - up to 32 bit / 96000 Hz

Specific Effects:
 - Noise Removal: YES
 - Tempo Change: YES
 - Change Speed: YES
 - Click Removal: YES

Real-Time Effects: NO

MIDI Recording: NO

Software Instruments: NO

Built-in Loops: NO

Overall Editing of Audio File: YES (Drawing and Envelop tools)

(GarageBand, 2008)

Things you CAN do with GarageBand '09 but
NOT Audacity

Besides creating and mixing audio recordings, GarageBand '09 also allows users to do the following...

 - Insert built-in loops, instrument effects, and sound
    effects into projects

 - Create new projects using the following templates:
        ~ Piano:
        ~ Electric Guitar
        ~ Voice
        ~ Loops
        ~ Keyboard Collection
        ~ Acoustic Instrument
        ~ Songwriting
        ~ Podcast
        ~ iMovie

 - Plays Mac like a musical instrument

 - New features on GarageBand '09:
        ~ Take basic piano and guitar lessons (tutorials)
        ~ Buy and download lessons from artists (learn how
           to play their songs) - each lesson can be
           downloaded for $4.99 from Apple's website.
        ~ New Guitar features (amps and stompbox effects)
        ~ Magic GarageBand (play with a full-screen band)

(Audacity: Free Audio Editor and Recorder, 2009; GarageBand '09, 2009)