Both can be oral, but flubendazole doesn't have to be. That's the advantage, if fish are not eating, it can still help. It's not available everywhere, many drugs are not.
I don't know if either are safe for shrimp. I mostly treat in QT without other fish or inverts.
Different drugs treat different suspected internal parasites. In desperation, I have used prazi only once. It didn't help my fish. I have used levamisole several times as a preventative quarantine treatment for loaches. It may have helped, or maybe they didn't need it. It seemed safe for them. I went with better safe than very sorry.
If we could tell what internal parasites we suspect, I think we could make better drug treatment choices, depending on country laws. It can be very hard to say. By the time we have a good idea what may be the problem the fish may be beyond help.