I only have a two year old iPod Touch and I have the Kotoba apps installed on it and I am satistied. It serve its purpose.
one nice thing about android, is that you can create your own application without submitting it like in iPhone apps, so you can basically tailored any application you want. So I think you can find more appli available on android, for free.
I think application for learning Japanese are readily available on both smart phone
But personally, Im old-schooled so I still like learning from text box
