Are you looking for audio quality? If you are, do not get anything apple. They are cut you short on audio quality, make sure you are downloading FLACs of the music, the files are bigger but you will notice the difference of audio quality, well if you have quiet nice audio equipment. If you are looking for a portable music player, do you have a smartphone, or one with a MicroSD slot? That would probably be the best for a portable music player, then you will only have to carry one device. But if you are worrying about size, I would go with MP4. When I have converted tons of content, I have run into some issues, like some of the content getting drastically reduced quality. I would recomend doing the converts in small groups. Also watch the temps of your computer when you do this, it will probably run your CPU at full load.