AA is fundamentally on SABRE and BA on Amadeus, the interface between them uses 1970s architecture and so has many limitations. So for keeping up to date with things like time changes, AA is best for AA metal, BA for BA metal. You may be able to check-in for the AA flight on BA, particularly if it is a codeshared flight, but it doesn't take much for it not to work, typically the AA app will then work. Essentially I use both, being a "guest" user on AA, and for any query I have I work out whether the answer is on SABRE or Amadeus, then use the apps appropriately. So if the AA app shows a cancellation and BA app does not, for AA metal the AA app is going to be correct.