Most of the time, I use my iPad as both a virtual amp rig (AmpKit+) and simultaneous backing track playback.

However, I also have an old 2nd gen 16GB iPod Touch that runs the exact same AmpKit app and can do the playback as well. You can get one of these 16GB models refurbished for about $130 or so, and not only do you have your MP3 playback with lots of storage, but a host of other options for music/games/other apps as well, all for a lot cheaper than the latest iPhone/iPod or droid phone.

Just a suggestion.