The difference between the two is that Bluesky has a central audience with decentralized content, while Mastodon has a federated audience with federated content.
Blueksy holds all the power, while the users hold none, whereas with Mastodon has many separate communities, similar to the old-school BBSes, forums, IRC and teamspeak servers.
Migrating to a different App View should be painless in theory (app views are not supposed to collect any state that is not saved in the PDS, not sure if Bsky does or not), and you can use multiple app views with one PDS. On Mastodon, you have to migrate both at the same time, and moving content across instances is not yet a fully solved problem.