Otos are herbivorous fish, they usually chow down on the diatom/brown algae but usually only eat microscopic forms of the others. So, baby algae bits, they help to keep algae at a decent pace so that it won't grow more and sometimes will rid the tank of it entirely.
Cories on the other hand are mainly carnivores, they are a schooling fish too and best not kept alone or in groups below 4. I have 1 corydoras elegans I saved from my uncles 10 gallon goldfish tank and plan to either add him to my mom's tank or get more of them for mine. I have fed mine shrimp pellets as a staple and they get the usual supplements like the rest of my tank, live tubifex and BS, frozen mysis shrimp, bloodworms and BS. They are also a cat that has soft barbels and if kept on gravel will wear them down and could open it up to secondary infections, so they really need sand or round gravel.
For your tank, I would recomend a couple of otos since they both stay smaller than cories and fit better in a smaller tank than cories do.