All arguments aside iOS has the advantage of a locked file system whereas Google broke Android down into constituent parts (ie apps) so that even people with really old devices (like 9 years) can still have secure apps without needing a whole new OS install.Found this out when I did a deep dive into a former Apple proposal:
Android you can improve security; iOS you are stuck with what Apple grants you.
End of the day, they are pretty well evenly matched if you take your time and set them up right.