Apple Difference Between Logic Pro, Logic Express, and Soundtrack Pro?

So Apple's website has a comparison between Logic Pro and Logic Express, but I would like to know where Soundtrack Pro fits into all of this.

I've been all over the place regarding Windows / Mac. I used Windows on all hand-me-down computers, then got a g4 powerbook, then needed something more powerful (I do a lot of video editing, and I needed something better than a 12" powerbook to do that) and wanted something to play games on, so I sold the powerbook and built a computer...but now I'm thinking of going back to Mac for mostly everything, and keeping my current PC for gaming.

I'd get the Final Cut Studio box set, but my friend also wants to use my computer to do a lot of audio production, so I don't know if Soundtrack Pro has enough features. Does anyone know enough about this to help me out, or even just guide me to a site to help? Google has just given me comparisons between the two versions of Logic, and don't mention Soundtrack.

Also, can you use Logic like Soundtrack, and export from Final Cut directly to that, and then back in?