On mobile apps, the users get logged out rarely as the login tokens work differently. So, i think it makes more sense to make the first screen for new users. So the first page, when they open the app should be to register.
Does that make sense?
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You can decide to show Login/Registration either based on data (first time use vs login token)
Or based on whether the app has a higher traffic web platform
Or based on expectation of the user via a bit of user research
BUT always keep the alternate option handy. Either with a tab-like switcher or a simple link.
Notice how the Facebook login screen shows the last logged-in user, an option to login as a different user and a clear button to register.
FIRST TIME USE: REGISTRATION
With mobile applications, it is possible to detect if the app is being opened for the first time.
So, in that case, it makes sense to show the registration page.
SECOND TIME USER: LOGIN
If the user gets logged out, you can detect that and show them the Login page instead. I don't have technical expertise in this, but I know for a fact that apps can store the username of the previously logged in user.