They're two different forms of diadromy. Catadromous fishes spend the majority of their adult lives in freshwater but spawn at sea. Anadromous fishes do the opposite; they spend their adult lives in salt water and spawn in fresh.
I've only had a peacock eel, but IMO they are wonderful fish to have. Mine has quite a personality and is really active after he gets used to his environment. I'd love to have a tire track or especially a fire eel some day.. but I guess I'll have to talk the wife into a bigger tank for that!