Yeah,
The M-Audio Transit seems the the more sensible choice for someone with a digital 5.1 system, since it has the Optical (TosLink) digital. But, again, the Theatre will give you digital output via Coaxial.
I think the only reason for the Sonica Theatre is for those who have an analog 5.1 or 7.1 system, since it includes the four mini plug inputs and the driver to go with it. But, thankfully, it does include digital, too, via Coax (RCA). Strangely, there is a Griffin product called Firewave, which is Firewire, not USB, but only outputs to analog.
As an added feature, I believe your Yamaha system can add two additional speakers, at a later time, to make it a 7.1 system without the need for the Sonica Theatre.
So, the Transit may give a tad better sound via the TosLink cable, and also let you keep the $30. You shouldn't need any "faux surround" adjusting via the M-Audio panel, once you've designated the "Digital" as your "Current Set", and then your decoder will give the "faux" treatment via ProLogicII and manual adjustments on the unit or through the remote, and you'll still be able to adjust through the iTunes equalizer, as well. The Transit will basically function as a pass-through. But, as Doug mentions, you can also use one of many "Speaker Sets" or "Custom Sets" via the M-Audio panel, but true Dolby encoded sound won't be deconed until you return to the "Digital" as a "Speaker Set", at least that's how it goes on the recent Theatre driver, as it's probably the same on the Transit. I've even decided to get a Transit myself, just to satisfy my curiosity.
You will undoubtedly soon have a great home surround-sound system, and that satellite radio sounds cool, too. But the Mac Mini is not known to have strong video output in some cases. So, do you have a proven strategy for the viewing part of DVDs through your Mini? Or, will you also use a DVD player somehow?
Peace to you,
C. Livingstone