Soundbooth is better only really in the number and quality of filters and whatnot you get. It may also support surround sound mixing, I can't remember. The interface is also maybe slightly more efficient, if you're used to it. Audacity, when paired with a decent sound-card and maybe an external four-track, if that's your thing, is probably up to most tasks, and if you've got experience with it, I say stick to it.
What exactly are you planning?