Firstly, you should spend some time reading the document called "Selecting a J2EE vendor" in the resources section on TheServerSide.com. It will teach you how to properly evaluate an app. server and make a better decision.
Then you can start by looking over the member submitted reviews here on TheServerSide.com. We have also linked to other reviews around the Net. Check out flashline.com's app. server information matrix (there is a link in the reviews page).
Once you have narrowed down your choices, post some questions in our forums so that people can help you come to a decision.
If that doesn't work, then consider hiring The Middleware Company to help you make your decision. You can rely on our J2EE expertise, we can evaluate your business requirements and help you choose an application server, and even provide you with consultants to get your project going.
Check out The Middleware Company
Hope this helps!