Well looking at how much each combination is applied in real life you could say that PHP Ajax combo rules but knowing that PHP is a lot easier to conquer and pretty much faster for development on simpler projects than that is the advantage it has. On the other hand we all know how strong Java actually is and it is only getting stronger.
Java hosts are getting pretty cheap, not as cheap as PHP ones but close.
Java has smoothly entered as one of the languages for the big enterprise applications where PHP has not made it yet. AJAX relates mainly to the front end of an application. I think that choosing between AJAX PHP and AJAX Servlets/JSP resumes to choosing the language for your application.