Allegiance box v box is the best way to go IMO, until they get a starter box out there. I'm doing Cult v Hordes demos because that's what I have, but I'd suggest having one of the Earthside Allegiances as well for people who like more "normal" armies to try.
When you're doing your demo, keep it basic. I don't have people use Stratagems until their second game, I explain that they exist and that's it for now. Just have people play a game and go over Orders, Actions, Duels, Cheating, using Tokens, and scoring VP. So basically the quick and interesting parts of what makes TOS unique, and if they show interest then you can mix up lists a bit, add in stratagems, and all that.