Convert .stmp file to MP3 file

I'm trying to convert a multitrack Soundtrack Pro (v 1.0.3) project into an MP3 file. The documentation for STP has nothing on this. The online manual for Compressor has a lot of marketing happy-talk about how AAC is better than MP3. Is this a religious issue? Is there no simple way to take a .stmp file and export/convert is as MP3?

Many thanks.