Although both options may be private. Traditional approach: if you have a pair of keys with a small key, use common preferences. If you have a large amount of data, refer to the internal storage. Internal storages are stored in files and are private to your application. The main disadvantage with a common advantage. if your application crashes / closes you, you will lose all your data in a general privilege.
Verdict:
If you use a common prefix. make sure your application does not crash. So my point is: go with internal storage. However, if your phone is rooted, anything can happen, nothing else remains private ...: /
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