Your choice of the device does not really matter when it comes to trading cryptocurrencies. What actually matters is the app and web platform that you are using. If the trading platform that you are using offers better functionality on the web, you will need a computer, most preferably a laptop because of its portability; however, the trading platform you are using has better functionality on a mobile platform, you will require a mobile device. Between Android phone or iPhone, or even windows, which device you should use also depend on the apps you are using for trading. If the app you want to use is available only on an iOS device, how can use trade on an Android phone, likewise, if the app you want to use has only the Android version, how can you use it on an iPhone? Now, you can ask what if the app that you use for trading purposes is available on both platforms, which device will be the ideal choice. Once again I am going to repeat the same answer, iPhne or Android really does not matter, you can use anyone depending on funds you have at your hand (for buying the device).