Like the ds, without the whytkin summon. Graphics have been sharpened for the ios, and they have added a difficulty option. If you still have/play it for the ds, it's probably not worth it. I traded my ds version a while ago, and was happy to pick it back up on the ios.
Thats what I though I was like "do I really need another version of ffII/IV?"
App Store is sort of like Steam now. Once you buy it, it's yours for good. So any time you miss Final Fantasy IV log onto the App Store.
Honestly unless your collector it might not be worth it. To me this is content I can always have at my finger tips. Plus the updated graphics make it more enjoyable to me. Though with missing content it doesn't help Squares rep