It is not a case of which is better, unless you can get a tank three times as big you will need to find another home for the plec. I think that a parrot fish would be alright in a fifty gallon maybe even a pair of them . However I know nothing about such fish.
Plecs are peacful and if this was a temoprary home till you found it some where else I think it should work. HOWEVER you must understand that to give this animal a good quality of life it needs a bigger tank.