Not to do the mod's job [:)], but to give you a quck answer:
The Orchestral Cube is a collection of single note samples along the lines of traditional libraries like Miroslav. There's a lot more though - dynamic controls, different playing styles, etc.
The Performance Cube is more emphasized on samples that involve more than one note. The major one is the legato - you will be able to naturally switch from one note to another with a realistic sounding legato, not the artificial pitch bend or cross-fade that is sometimes employed in synthesizers. Apparently to get a legato you play the primary note normally, then pick the next sample based on what the previous note was. They recorded samples of all the instruments going to/from almost all conceivable notes in their range to do this. Quite a monumentous feat. I can't wait to hear demos!
Mods, which reminds me, it's ok if you don't know yet, but do you know when the demo cube will be released? You'd mentioned November I think. I'm waiting with baited breath. Also I live in the U.S. - I hope it won't take very long to get here!