You could use both of them.
RxKotlin contains an extension functions that provides to you idiomatic way(with writing less code) of using rxJava in Kotlin.
rxAndroid(is now split in few libraries) provides specific platform bindings such as:
- rxAndroid that could help you to manage an execution with native android Handlers
- rxLifecycle that provides Activity lifecycle bindings
- rxBinding and rxBinding-kotlin that helps to manage UI with all power of rxJava
There are few more useful libraries for rxJava and Android you could be interested.
Edited 01.04.16