iPhones are secure, maybe they contain backdoors from Apple and we don't know, but Android are not secure at all, especially because most Android vendors usually don't update the OS to the latest security patches, so the majority of the Android phones out there are full of unpatched security vulnerabilities.
Also there are not good free software mobile operating systems, sure there is LineageOS and other ROMs that still require some proprietary parts, mainly the firmware of the device and binary blobs in the kernel, for one person concerned about privacy that is a problem, because proprietary software means backdoors, and it's useless to use a fantastic free software secure communication app if we can't trust the OS where we run it.