DISCLAIMER: I work for Oracle as a product manager for TopLink.
The customers that I deal with who have compared both and selected TopLink or moved from Hibernate to TopLink tend to list eth following in their reasons.
1. PERFORMANCE: I will start by saying that every application is different and performance comparisons are difficult to justify in a generic sense. In our experience TopLink's shared object cache is a true differentiator. The ability to configure single node and multi-node cache coordination on a per type basis allows for much more optimized access to many of the persistent types in your application. When running comparisons you will need to spend some time to learn both products. Using either without optimizing for that products recommended usage is not giving you a true comparison. You will also want to be sure not to fall into the counting SQL. In many of our comparisons with other ORM products we have found in many cases controlling the SQL in critical to performance and large multi-tables joins are not always as fast a two or more queries returning less data.
2. PRODUCTIVITY: TopLink provides meta data configuration through API and XML. For the XML configuration the mapping editors (JDeveloper and Standalone with any IDE) are not simply XML assistants they provide contextual configuration which is not only productive when mapping but also very helpful during iterative (agile) development. As either of the models (object or relational) are changed all related mappings and graphically defined queries are flagged with warning or errors. This is much more efficient then finding all of the effects of your changes at runtime.
3. COST: This is always a challenging one as many out there consider open source to be free. In reality all external software used in a project has a cost. TopLink's is license based and JBoss Hibernate's is purely services (consulting and support) based. Those of your looking for to compare apples and apples should consider the full cost of having a professionally supported product in both cases. I am sure the JBoss folks would want you consider purchasing their services as well.
4. ORCLE: TopLink support all major J2EE application servers and Databases providing testing and certification on all releases and patch-sets. In addition to supporting these common heterogeneous environments we also strive to provide the best possible support for the Oracle stack. On the Oracle application server we provide the container managed persistence for EJB2 and EJB3 in our container as well as tight integration for security, logging and management, etc. For the Oracle database we provide support for many advanced features of the Oracle database including TIMESTAMP* types, Native Sequencing, N*LOB, XDB's XML Type (mapping and SQLX querying, Object-relational types, hierarchical querying, hints, stored procedures & functions, and Virtual Private Database (VPD) to name a few. Many customers find leveraging the advanced features of their database key in their ORM selection.
5. EJB 3.0: Oracle's early adoption of the EJB3 persistence API and delivery of an initial preview have also been listed among customer's selection criteria. We are strong committed to this standard and providing early access as leading the tooling development within JDeveloper and Eclipse to assist in this adoption.
I hope this helps in your comparison and selection. Some additional resources to help in your research:
TopLink's home page: http://www.oracle.com/technology/products/ias/toplink/index.html
Oracle EJB3: http://otn.oracle.com/ejb3
TopLink's Discussion Forum: http://forums.oracle.com/forums/forum.jsp?forum=48