the eel has more character from what I've seen (though I don't own one). If you have a sand bottom, it's interesting to see them poking through the sand in one place and then the rest of their body somewhere else like a sea serpent. Though, the only con I can think of is that they get really big, have razor sharp spines, and are the ninjas at the art of escaping.
The black ghost also gets really large and is full of character (also, dually noted that I do not own one), but may be timid when younger. Knifefish also enjoy a sandy substrate to perform their natural feeding behaviors. Oh, and they're also nocturnal fish, so the most interesting behavior will usually occur once the lights are off.
Can't go wrong with either IMO as long as you have a large enough tank.