IOS is better secure because every app on the Apple App Store is closely inspected, which might reduce the number of apps available, but helps to reduce malware-riddled apps and security issues.
For android apps open google play Store marketplace has far more apps to choose from then on and the apple App Store, but there’s a far greater chance that hackers can make it onto the platform to distribute malware through malicious apps. On numerous occasions, highly ranked apps with hundreds of thousands of downloads from the Google Play Store have been discovered to contain Android malware.
Android users can also change their settings to allow apps from outside of the google play Store. This provides an even greater selection of apps, but opens users to an even greater risk of downloading a malicious app. Apple’s secure App Store wins comfortably on this front. As a side note, make sure to avoid third party app stores; while the Play Store and the App Store have their faults, they’re both much safer than the many unofficial app marketplaces elsewhere online.