iOS
In terms of UI (user interface), iOS seems to be the most simple, intuitive, & consistent among various devices whether they are smartphones or tablets. On the other hand, because of the simplicity and probably to ward off users from screwing up the UI, the OS is not very customizable in comparison to Android — you can’t change the dock or skin. What irritates me quite a bit about iOS is that you cannot copy files or folders from a file manager or explorer (e.g. My Computer in Windows) directly into the internal storage of an iPhone or iPad. Instead you’d have to sync through your iTunes library or transfer through a 3rd party app.
Android
Because different smartphone or tablet brands customize the UI and other features of Android, the UX (user experience) is not consistent across devices in terms of usability, performance, stability, & other features. Oftentimes to various degrees by mobile device brands, preinstalled bloatware is bundled with Android and many are not uninstallable without rooting (like hacking) your Android device.
Conclusion
I do prefer Android more mainly because I can customize the UI to my preference unlike iOS and the greater freedom and ease of tweaking features. However, I’d recommend iOS for people who put a huge emphasis on simplicity and are very accustomed to the iOS and iTunes Ecosystems.