Android is open source and you can do what ever you want if you know what your are doing. IOS however does not have the freedom that Android has. But what is best for you? Here is a really quick look on what might be best for you.
If you want something simple to use and you don’t plan to “play” around with it and customize it that much, get an IOS device.
If you want something highly customizable, get an Android.
If you want something good for texting and calling, get a flip phone.
There I solved all your problems.