Just to add to that, we were working on a WebSphere 6 project up until a few weeks ago when the client compelled a switch-over to Weblogic 9.1.
I found WebSphere slightly easier to configure, and IBM's on-line documentation is a whole lot easier to get answers from. On the other hand, WebSphere is stuck in Java 1.4, while Weblogic comes with two Java 5 runtimes, so you are free to use all those kewl J5 features.
Given the choice I still vote for Resin, which now comes with built-in PHP support
If you are looking at the IDE too, note that RAD (rational application developer) 6 is what goes with WAS (websphere application server) 6. They renamed it from WSAD (websphere studio application developer) to RAD.