Android is a much better choice for chess. For one thing Apples iOS makes it difficult to have files on the device. I've had both and currently have a iPad and Samsung S6 phone. Apps I have are Chess.com, Scid on the go, Chess-wise is good for both Android and iOS, Analyze This, and Chessify for grabbing positions out of books. There are lots of free or low cost engines like Komodo, and Stockfish.
Chess Engines on Phones - iPhone/Android

Hi everyone. I currently spend a lot of time travelling on the bus and want to use this time to study my chess. I am thinking about buying a smartphone and studying with engines such as Stockfish 8 and Komodo. I have a few questions and I would be grateful for any assistance
1: How much functionality do the phone versions of Stockfish 8 and Komodo have on iphones/ios and android ? Basically can you do all the same things that you could do on a computer ? Related to this, what good GUI/UCIs are available on different smartphone operating systems ? Preferably for free. Which interfaces are good for loading multiple engines ?
2: Does Smallfish use an optimum version of Stockfish 8 ?
3: What general reccommendations do you have ? Should I buy an iPhone or android phone for easy use of top engines ?
Many thanks in advance for your help