It probably won’t make much difference because both are fast. The internal will likely be faster than any external, however, I doubt you’ll see a difference.
One advantage to keeping the libraries internal, is that if you are running Time Machine, you get a back up automatically. However, the same can be achieved through other methods. Regardless, I would recommend the 1TB SSD so you’ll have space if needed.
In addition, I would recommend against bringing the media into the library. Leave it linked “in place”. In that case, your media might be on the external SSD and the FCPX library could be either internal or external. If external, at the end of the session, you could also copy the Library to your internal drive as a back up. Or the other way around.
I personally have libraries on both internal and external drives. Of course, if the library is on the same external drive as your media, then the project is portable if you need to use different machines.
– Oliver
Oliver Peters – oliverpeters.com