My answer is a little fuzzy, since it’s been years when I even touched stuff like this. I was hoping someone else would have a better answer. However….
I believe that a link encryptor is used to encrypt a particular wire/fiber/cable connection – point-to-point. For example, say the U.S. Pentagon to some other military facility, so that if someone were to tap the cable, all they’d get is gibberish – AND more to the point, the end users would neither know nor have to care about security – it’s been taken care of. They may well put their own security on top of that (such as TLS).
The difference is that the link encryption is typically taken care of by the people who maintain the external connections.
If you’ve got a serious enough need to consider both, then do it – and have them administered by different groups – each accountable in their own right.