Pimento loaf,
What do you want to accomplish with the app? How do you expect to use it and what results do you need?
Soundtrack Pro will let you record, edit, multitrack and automate audio and loops. However, there's no MIDI input or software synths, so all your source audio has got to come from either another app, or from a sample loop library, for example.
Logic Express will also let you record, edit, multitrack and automate audio and loops. THis time, there are built-in software synths and the ability to add third-party synthesizers, samplers, etc.
Best,
Anton