Should I use EJB3 or Spring for my business layer?

There won't be much difference between EJB3 and Spring based on Performance. We chose Spring for the following reasons (not mentioned in the question):

• Spring drives the architecture in a direction that more readily supports unit testing. For example, inject a mock DAO object to unit test your business layer, or utilize Spring's MockHttpRequest object to unit test a servlet. We maintain a separate Spring config for unit tests that allows us to isolate tests to the specific layers.
• An overriding driver was compatibility. If you need to support more than one App Server (or eventually want the option to move from JBoss to Glassfish, etc.), you will essentially be carrying your container (Spring) with you, rather than relying on compatibility between different implementations of the EJB3 specification.
• Spring allows for technology choices for Persistence, object remoting, etc. For example, we are also using a Flex front end, and are using the Hessian protocol for communications between Flex and Spring.

The gap between EJB3 and Spring is much smaller than it was, clearly. That said, one of the downsides to EJB3 now is that you can only inject into a bean, so you can end up turning components into beans that don't need to be.

The argument about unit testing is fairly irrelevant now - EJB3 is clearly designed to be more easily unit testable.

The compatibility argument above is also kind of irrelevant: whether you use EJB3 or Spring, you're still reliant on 3rd party-provided implementations of transaction managers, JMS, etc.

What would swing it for me, however, is support by the community. Working on an EJB3 project last year, there just weren't a lot of people out there using it and talking about their problems. Spring, rightly or wrongly, is extremely pervasive, particularlty in the enterprise, and that makes it easier to find someone who's got the same problem you're trying to solve.

What are the arguments for or against EJB3 vs Spring?Spring is always innovating and recognizes real-world constraints. Spring offered simplicity and elegance for the Java 1.4 application servers and didn't require a version of the J2EE specification that no one had access to in 2004 - 2006. At this point it is almost a religious debate that you can get sucked into - Spring + abstraction + open-source versus Java Enterprise Edition (Java EE) 5.0 specifications.

I think Spring complements more than competes with the Java EE specifications. As the features that were once unique to Spring continue to get rolled into the specification, many will argue that EJB 3 offers a 'good enough' feature set for most internal business applications.

What pitfalls can I expect with each?If your treating this as persistence issue (Spring+JPA) versus EJB3 your really not making that big of a choice.

Where can I find good benchmark information?I haven't followed the specj benchmark results for sometime, but they were popular for a while. It seems that each vendor (IBM, JBOSS, Oracle, and Sun) get less and less interested in having a compliant server. The lists get Shorter and shorter of certified vendors as you go from 1.3, 1.4. 1.5 Java Enterprise Edition. I think the days of a giant server that is fully compliant with all the specifications are over.