Exodons are beautiful fish, but you are severely limited in what you can keep with them. I have seen them used in African Cichlid tanks as "dither fish" and they can hold their own to be sure and seem to tolerate the hard water. You have to keep either one fish or quite a large group of them to spread out the aggression, have some thick planting for any injured or weak individual to hide, and prepare for some losses. If keep below a certain threshold number (12 or 15 might be a good guess) they slowly kill each other off. You can keep fish like Blue Botia with them or heavily armored catfish like Plecos, but in general then make life miserable for tankmates.
Congo Tetras on the other hand are generally cheaper, they are peaceful but very active, they do show some aggression within there own species but nothing serious. I have a school of Congos (12) in a 180 with Black Ruby Barbs, one male American Flagfish, Siamese Algae Eaters, Corys, and some Ottos. I may eventually make it more of a Congo River habitat, but compatible Congo River fishers are difficult to find locally.
Given the choice of the two, I would go with the Congo Tetras because they are more versatile in what you can keep with them.