Your answer lies in what type of music you want to write on the iPad. The whole song project needs to be resident in memory. You can make music with 16 GB, if you are not doing tons of tracks of audio. If it's mostly midi with soft synth (not sampler) instruments, you can do pretty large scores with 16GB of memory. As soon as you start using big sample libraries or lots of audio, then all of those items have to always be resident in memory.

The question is more like how much can you afford.

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