You should use all 3 sub formats because you will find that android os is running on devices that support all sub-formats.

Complete test cases and samples here : http://jacksondunstan.com/articles/2013

All sub-formats. All mip-map levels.

png2atf -c -i image.png -o image_all.atf

All sub-formats. No mip-map levels.

png2atf -c -n 0,0 -i image.png -o image_all_nomip.atf

Only DXT. All mip-map levels.

png2atf -c d -i image.png -o image_dxt.atf

Only DXT. No mip-map levels.

png2atf -c d -n 0,0 -i image.png -o image_dxt_nomip.atf

Only ETC. All mip-map levels.

png2atf -c e -i image.png -o image_etc.atf

Only ETC. No mip-map levels.

png2atf -c e -n 0,0 -i image.png -o image_etc_nomip.atf

Only PVRTC. All mip-map levels.

png2atf -c p -i image.png -o image_pvrtc.atf

Only PVRTC. No mip-map levels.

png2atf -c p -n 0,0 -i image.png -o image_pvrtc_nomip.atf