I predict that the Trump town hall will get bigger ratings than the Biden town hall. If I'm right that Trump will brag about the ratings "win" afterwards under the assumption that it means that he has more support than Biden. But actually more people will watch Trump because it's just more fun to watch demented lunatic give a train wreck of a town hall than a regular town hall with a politician trying to give normal answers.
UPDATE (10/16/20): I was wrong. Biden ended up ahead in the ratings, even after you add up the viewership of all the NBC channels (NBC, MSNBC, and CNBC), the viewership of the Biden town hall on ABC was ahead by about a million viewers.