Maybe you can satisfy both yourself and your customer :) Have a look at
'Quercus' technology by resin.

See : http://wiki.caucho.com/Quercus
and : http://www.caucho.com/resin-3.0/php/index.xtp

it is hard to compare both but in general

PHP web servers mostly run PHP by interpretation, but JSP pages are pre compiled, some commercial application servers like Bea weblogic optimize JSP to a very good speed.

there are not many high speed PHP containers.

it is harder to maintain a large PHP based  application than a j2ee  application

we are also developing some e-university solutions in our company, we have developed some systems for different universities one of them has 10,000 students and they have no problem with speed of our java application.

This link may help you:
www.sitepoint.com/.../

I totaly disagree with it, for example take a look at this part:

When it comes to session data I'm much more willing to believe in filesystem or database clustering than J2EE clustering, the key points being maturity both in the mechanism and the tools that support it (for sysadmins). There's also some serious weight going into Linux clustering which presents another path for PHP while other alternatives like MSession or memcached (pecl::memcache) also exist

do you really think, it is really a good idea to use a shared filesystem or database to store distributed http  session and application scope variables?