In-app version might require an internet occasional to verify the subscription. This is completely transparent to you. This might cause a problem if you have an Apple TV that isn’t connected to the internet.
The other different is you get free major version upgrades when they come available.
Apple does verify in-app purchase and subscriptions sometimes, but this is usually done when installing an update, which requires you to have internet anyways.
If you were to use the device 100% offline, you should’t run into any trouble with the lifetime license. With a monthly/annual subscription, you would need to connect once in awhile so Apple can let the app know your subscription is still active.