Swiftkey is winner for iOS hands down.
Next Keyboard will be releasing soon but from their beta, it has a long way to go in terms of predictions and smooth swipe typing.
For Chinese typing is the same as Android's SwiftKey. Not supported, have to use the default Chinese keyboard which is very good imo even with handwriting feature
To switch keyboards is much easier than on Android too.
On SwiftKey you can choose to use Emoji button or Keyboard button on settings, if you choose the former you can access the keyboard settings by holding onto the Emoji button, vice versa for if you choose the keyboard button, hold it and it will go to emoji.