The description on the App Store indicates that the iOS version includes some content previously exclusive to the DS version. Is it a direct port with tweaked controls, or are there other differences?
According to Wikipedia, it is a port of the Nintendo DS version, with "graphics optimized for iOS". It sounds like it is identical to the DS version from a gameplay perspective.
The IOS version lacks the Anime Cutscenes found in other versions of the game like the DS one and I think the PSX. However the DS version has additional content the PSX release doesn't have. Although the differences are mostly with the end game. I have heard that the IOS and Android versions have some form of DRM. I don't know if it requires you to always have internet access like SOME games.
it has some of the things that the DS version does make sure you have a newer iphone/ipad as well. The biggest complaint is probably the touch controls even though you're able to move in 8 different directions. There's also additional weapons or items listed in the DS FAQs which can't be found in the IOS version. (unsure if this is the case couldn't find secondary source) The IOS version includes the two new pieces of content that the DS had. But it doesn't inlcude the Arena of Ages Mini-game which has so far been a DS exclusive.
At the end of the day the games are the same look up some footage of the minigame and if that appeals to you and if you like Anime Cuscene's then get the DS version. The IOS version is more convenient since you always have your phone, alt