The Fall Update for 2019 is just a cumulative update to 1903, not that different than the monthly variety. It does depend on having 1903 already installed -- otherwise you will get the full version upgrade with a complete new version of win10 installed. Some are comparing the fall update to more of a Service Pack, but at least so far, the update, KB 4508451, released today to the Insider Slow Ring is a bit small for that. Remember that the Spring 2020 upgrade is where all the major changes are happening -- *might* not be a bad idea to get the latest before that version's out since it will likely not work for a higher percentage of people.
There's some speculation that this may be the new way that Microsoft upgrade win10 from now on, with one major & one minor update... IMHO it depends on what they can get away with, in line with Microsoft's statement that they're trying something new.