ktrack: What's the difference?

Whilst they share a number of core features, Soundtrack and GarageBand are targeted at different markets and have been designed for different purposes.

Soundtrack was released before GarageBand. Soundtrack is primarily a loop based composition tool intended for use in scoring video, film etc. It is a "Pro" application. GarageBand is aimed more at the consumer market.

The Key Differences:

*Soundtrack can import, play & sync to a video clip, GarageBand cannot.
*Soundtrack can export directly to stereo AIF, GarageBand must route all exports via iTunes.
*Soundtrack can export separate AIFs for each track in a project, all in sync and with any mix automation, effects etc. applied to each individual track. This is useful for taking the separate tracks into another application for further manipulation. GarageBand can't do this.
*Soundtrack can work at higher bit depths and sample rates than GarageBand. GarageBand is limited to 16 bit / 44.1 kHz (i.e. CD quality).
*As of v1.2, Soundtrack can natively sync or slave to other MIDI devices, enabling them to "play" together in real time. GarageBand can't do this, however there are 3rd party utilities that aim to fill that gap ( MidiO enables GarageBand to control other MIDI devices, GarageRemote lets other devices control GarageBand via MIDI).
*Soundtrack ships with over 4,000 loops in the box, including many ambient/FX loops (doors, steps, sirens, nature etc.) useful in scoring for video/film. GarageBand ships with about 1,000.
*The one thing GarageBand has over Soundtrack is the ability to record midi sequences in the timeline, using either the Mac keyboard or a 3rd party MIDI controller, and play them back using virtual instruments.

Irrespective of their differences, either application can be used to create music in a wide variety of styles.