FrankYan in my opinion you should use highest resolution possible.
At my project, person responsible for graphic prepare assets for iPad4 resolution (2048x1536).
And for the lower memory usage we prepare 2 atlases: HD (2048x1536) assets and SD (1024x768) assets.
And the SD assets are prepared automatically - just resize 50% the HD assets.
After that when loading your application check the devices resolution and determine which atlas to use.