Choosing an Enterprise Java Application Server

With your wisdom and experience, you can help us choose a new application server platform! We are in the process of choosing an application server to host a Enterprise Java application that is custom build for us by a system integrator. The application has a typical web application architecture: accepting TCP and HTTP/S requests, doing business processing, updating an relational database system and sending email. The application is running 24 hours 7 days a week.The application is deployed onto a cluster of 4 machines fronted by a F5 appliance, for availability and load balancing purposes. The whole cluster is monitored using Wily Introscope. The rollout of new releases is normally scheduled in a maintenance window. Maintenance of the application is done in-house an a full Development, Test and Acceptance environment is available. Operations and systems management is done by a different group, than system development. The application is currently running on BEA WebLogic 8.x to our complete satisfaction. As this version is end of life, we need to choose a new platform. The source is ready for deployment on J2EE 1.4. Instead of listening to the (pre)sales of the vendors, we would like to learn from your experience and listen to what you would advise.
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