There is no official sap position on this question as far as I know. So I will throw my two cents. First of all, I don't know of any good argument to use Java over ABAP in sap land.
It seems to me that new thing were developed in Java by Sap, because it was the "future". It looks like they actively hampered the development of the ABAP stack to give the Java stack a chance. But the only reason to use the SAP java application server is to access information and business logic on the ABAP application servers. There is no reason to use it by itself (as there are better j2ee alternatives). And even sap has not done any application of significance on the Java Application Server.
About the maturity. The webdynpro are more mature on the java stack. But the stack itself is terribly imature.
It take more than ten minutes to start on a decent hardware.
The logging is scattered all over the place. With unusable gui. And very unfriendly to text viewer.
It is not using the java community resources for staff like version management and builds. And the SAP alternatives are imature.
About the availability of java programmers. That’s true, of course, but: Since most of the logic is in the abap stack ( in most scenarios ) there is a need to know both java and abap for any, more than trivial, application. And here you are dealing with a scarce resource. What happens is that you teach abap to java developer or vice versa. This is not bad but it is done from the wrong reasons. And you will have inexperience developers on the project ( inexperienced in one language stack).
Final word. If you care about usability of your site. Consider BSP or JSP. The webdynpro revive all the ugliness and un-usability that is in the Sapgui.