What is the best AIR render mode?

In Flash Professional Help / Publishing AIR for Android applications (http://helpx.adobe.com/flash/using/publishing-air-android-applications.html) I found the following:

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Render mode

Allows you to specify which method the AIR runtime uses to render graphic content. The options include:

    Auto - automatically detect and use the fastest rendering method available on the host device.

    CPU - Use the CPU.

    GPU - Use the GPU. If no GPU is available, the CPU is used.

    Direct - Render using Stage3D. This is the fastest available rendering method.

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If the Auto mode automatically selects the fastest rendering method, what is the point of the 3 other options?

My application has some minor 2D animations, but very smooth drag-and-drop performance is important. What rendering mode would you recommend?

Thank you.