what frame-rate should I choose for iOS devices ?

Yes release is same as non-debug

Okay I finally get your question.

The idea is to keep a value for fps that meets your requirement. Using a value higher than this wastes resources. A simple example would be running an application at 60 fps that is an enterprise application which is clearly wasteful. For your scrolling usecase, if your performance requirement is of 40 fps then keep framerate at 40. Now two things can happen either your device will have enough juice to run at 40 fps or it won't have. If it has the juice it will run at 40 fps no issues. But if it does not have enough juice then it won't and cause stutter. In such a case raising the framerate to 60 will serve no purpose, because that relatively slower device could not scroll at 40 fps, no chance of it to scroll at 60 fps. You should not concern yourself to keep different framerates for different devices. Keep the framerate at a value that meets your requirement, if the underlying device can play at that fps it will play, otherwise it will stutter.

To answer your original question, no you will not get smoother scrolling on a lower frame rate.