Its hard to say which is the best for each scenario but I'm sure I can give some pro's and cons for the loading indicators you specified.
- Pros: More discreet. If the app has information on screen, and is loading more info this method may not grab the users attention and possibly allows them to view information that is already available.
- Cons: More discreet, user may not be aware that app is loading information or the user may think the app is slow or not responding
Middle of Screen
- Pros: Because its in the middle of the screen, the user is aware that app is loading information.
- Cons: If loading more information, the loading indicator will take the users attention or possibly prevent the user from viewing information that has already loaded.
Like I said, it's hard to know what is the best option for your app without knowing the different scenarios that require some feedback to the user that the app is loading information.
Luke Wroblewski wrote an article about Avoid The Spinner which I would recommend reading.
Anyway hope this helps! 👍