Initial tests on a Sharp Zaurus shows that the fixed-point decoder works its magic with a significantly lower amount of CPU overhead than mp3's, but your mileage may vary.

I don't know which mp3 decoder theKompany uses in their tkcPlayer app, but on that unit, the Vorbis fixed-point decoder took about 25-35% usage, while mp3's took 35-45%. That's on a 206Mhz StrongARM, but you may get different results. We'll see. My educated guess is that it'll take less firepower to decode oggs than mp3's on the Empeg, but it's just a guess.