in our test suite we are using both WebDriver and Graphene drivers.
Each of them has its advantages, therefore we have two super classes from which the test inherits, because currently you can not have these two drivers simultaneously in one test class.
Graphene advantages can be read here. In short it provides for us lot of utilities which are veru handy with testing of AJAX applications (Ajax guards, interceptors, XHR halter), we like its strong typing features and other.
We use WebDriver driver for testing mobile sites, as that is currently possible only with that.
However, Graphene is undergoing lot of improvements. There will be 2.0.0.Alpha2 release with many new features. The main is, that it integrates Graphene 1 best features together with WebDriver support. Please check arquillian.org for release notes later this week.
Feel free to ask any more specific questions.
Thanks a lot, this is typically the sort of definition that helps choosing one or the other.