I have used the Theater and the regular Sonica in the past. Both connect to your Mac using USB and you need to use M-audio's driver. The Sonica has optical digital (toslink) output, whereas the Theater has multiple 1/8" sockets and a coaxial digital connector.
What do you intend to do with it? Both will allow you to pass through Dolby Digital and DTS signals to an AV receiver, when using Apple's DVD player or VLC. Besides that, I believe you won't get more than PCM stereo out of the digital connectors, but you should be able to get more channels out of the 1/8" jacks, but no Dolby decoding.
Hope that helps...