I just compared the 2 apps on the app store. Seek claims the only data used is location and media (images/video). Merlin links identifiers, location, usage data, and contact info.
So, one is either not listing all of the data they harvest from you, or one is a really cool app. I have a feeling both apps are not being fully honest.
They are both run by charities unlike some of the apps in this field. Seek is aimed at children and so collects as little data as possible. It can upload to iNaturalist but only if you connect accounts. Merlin doesn’t collect the data unless you create an account either I don’t believe.
So, one is either not listing all of the data they harvest from you, or one is a really cool app. I have a feeling both apps are not being fully honest.