You will have to decide what you want. Do you like to watch cichlids rearing fry? If so a 250 would be a great tank for a pair of RTMs. Would you rather have a monster pet? Less stress (for you) not having to worry about weather your male has murdered your female, well, there is something to be said for that. Do you get bored by the sight of a single fish in a tank like that?
For me, I`d go big, but then I have never had a cichlid larger than 16" and it would be a real treat to have a beast like a huge umbee.