Here I have outlined four scenarios that could transform into reality after a hard fork.
For me, the worst case would be a "permanent indecision" scenario or at least indecision for several months. That would be the case if none of the two implementations could score a clear "win" against the other one. Market cap would be divided (at least, more than a third for one, less than two thirds for the other).
I think this scenario is not totally unrealistic if we take into account two things:
1) many players of both sides would try to "dump" the other currency to accelerate a win of the "own" chain. That would favour Bitcoin Core, because the economic majority is there, as far as I know. So termporarily, first the BTC price will be well over the BTU price.
2) but miners that have decided to adhere to Bitcoin Unlimited won't change back to the Core chain that fast after achieving 75% of the hashrate, because they would lose money. That could lead to the BTC blockchain to get slower and less usable until the next difficulty adjustment. In this case, a part of the "apolitical" users - above all, merchants - could change to BTU. That would mean the "economic majority" assumption for Core would not be that clear anymore.
Why is it a worst case? Because none of both coins could emerge as a clear leader and group together the network effect advantage Bitcoin currently has with respect to other coins. That could lead to an erosion of this advantage.
I for myself consider a scenario where Core retains more than 75% of the economic power more probable (a scenario similar to the "ETH/ETC" fork) but this worst case scenario should not be ruled out.