Short answer - Neither
Long answer - Both of them have an inherited structured which relies of market management (more like manipulation) by largest holders - the fight to fix price to $1.
bitUSD does not depend on market "management", only people competing to make a profit. It assumes people act greedily and are not "fighting" any market forces (any deviation is an opportunity to trade profitably).
Here's the thing if people act "greedily" they should be acting greedy about their BTSX holding too. That would mean net shorts will always be higher than net longs - isnt that the case in the current market? There have been quite a lot of restrictions put in place to not allow people to short. Something like - you cant short below 5% (not sure) below the MA price or interest bearing etc. So BTSX on one hand believes people to act rationally (greedy) but disallows people to act on their own accord (based on their greed). How is that a good thing?
Problem with NuBits and BTSX is same, free market reign is restricted given, while talking about the market is free. Not a good combination.
Restricting shorts based on a price feed is not "restricting the free market". When you enter a contract for a difference, you and the other party have to agree on a real-world price.
Not having this restriction would be like forex allowing you to do naked shorts at a price other than the current market price for the real asset.