Speed really won;t be the factor here, it is the same speed.
You will get more features with Toplink APIs that are not implementation of the JPA spec. However, with specific APIs you become more vendor locked. But that occurs with all ORMs that implement JPA, they also have vendor specific APIs that you can use to get some extra features that the JPA spec doesn't cover.
But if you stay with the JPA spec, then you can swap out the vendor easily with any other JPA spec compliant vendor.
Personally, I would take your choice of JPA vendor, and use as much JPA spec compliant code as you can and only use vendor specific code when necessary. Sometimes you can even quarantine the vendor specific code in helper classes, but that is up to your architect if you want to take that approach.
Mark