The offline approach focuses on the highest security available by storing your secret on a truly offline device with no connections to any network whatsoever. Ensuring that the secret is kept as safe as possible but with a caveat that the QR communication might be more cumbersome to use. This should be used as cold storage of larger funds.
The same devices approach brings still a lot of added security in comparison with other mobile wallets. With the two app approach, the secret is encapsulated and protected through the permission handling of the operating system e.g. no network permission for AirGap Vault. The communication between the two apps is easier to use thanks to simple app switching. This is intended to manage smaller funds or used to interact with decentralized applications.