I utilized the "bonito" images for Pixel 3a XL for the below comparison.
Within the .19 image META-INF\com\android\metadata, the post-timestamp UNIX value is 1565300664 which equates to 08/08/2019 @ 9:44pm (UTC).
Within the .20 image META-INF\com\android\metadata, the post-timestamp UNIX value is 1565743352 which equates to 08/14/2019 @ 12:42am (UTC).
Based on one of Google's release tool's code, this value seems to be utilized to determine whether it should perform an incremental or full OTAs.
If I'm understanding their upgrade process correctly, the .19 image does not contain the September security patch. The .20 appears to be the full stable release with the September security patch.
It would appear .19 should support an incremental upgrade from the latest Android Pie (9) version, PQ3B.190801.002 but not bring you to the latest September security patch.
This leads me to believe that the .20 image is suitable for almost all OTAs. There may be carrier specific cases that one or the other should be utilized but nothing stands out too clearly within any of the immediately available metadata files.