Other then mixing 2 types of eels is potential for disaster. They usually do fine as juviniles but once they mature it's on like donkey kong. my 3 fire eels are bickering with eachother at the moment and I hope they figure out a workable solution. But I am prepared to seperate if need be. so if you do try mixing make sure you can seperate them if you have too.
Other then the fact a TT is more likely to stay within the size of the tank either species is capable of outgrowing it, which you know. My TT is now no less friendly then my fire's are. He's handfed earthworms but also eats feeder guppies, where my fire eels eat worms, and more worms, and now the occasional flake... weird fish.
as long as your prepared to accomidate their special needs either eel imo is equally attractive/personable. It's just as the owner how much attention you put into it. and the fishe's individual personality.
Mine are deffinately my favorite fish now. It also seems liek the TT's grow alot faster then the fires do on average. then taper off at a younger age then the fires do. I haven't had my TT long enough yet to really say for sure. BUt I'm sure he'll outgrow my fires as he's about 6" and they're in the 12" range. But in the long run I'm betting my fires will get larger.