Probably native OS performance + battery life (depending on which Android device one has it can be good [Nexus 7] to mediocre [Verizon Galaxy Nexus]). Perhaps they prefer Cocoa + Objective C to Java, if they're a developer. It's a tradeoff I suppose to which one considers more important. I like Android well enough these days and try to use Mono + C# when possible, but each platform has its merits and have to weigh what is more important as a user/developer.

disclosure: Android user for nearly 3+ years and owns a Galaxy Nexus, Nexus 7 and a few other Android devices.