How similar are JPA and Toplink such that expertise in one could carry over to the other?
As I understand it, TopLink is an implementation of the JPA standard. There are others as well, and using TopLink implies using JPA. Of course with the newer versions of the JPA standard, I would suggest using a more robust architecture such as Hibernate. The JPA standard infact borrowed quite a bit from Hibernate, and so now they are inline with one another. Another advantage is that there is a lot of reference material online for Hibernate, and TopLink is nowhere close to this.
Hibernate is a pretty complex tool, but experience in it would be highly desirable in any Java shop.