The NBT peg is clearly much tighter than the BitUSD peg. I believe this is because NBT relies on people/pools to maintain it, while the latter depends on market forces that are less reliable.
In essence, the new v2.0 “SmartCoin” BitUSD is completely overhauled from the earlier version. I think it has a stronger lower bound, but the upper bound is not as tightly enforced. We’ll have to wait and see how well it works in the real world.
EDIT: I think more official documentation is likely still to come. I’m not a BTS delegate anymore, and I have the same information as the rest of you. We’re all in the same boat.