I have an Oscar. He/She was my first purchase. I wanted something big and that would make an impression when others walked in the door. I love him. He has the greatest personality. Like a puppy dog as someone else mentioned
I have never had a Dat so I really can't say much. I love the way they look, but I've heard good and bad of them. They are expensive when big and unstable when purchased small, so I've hear.
I think Dats are for more advanced fish keepers only because they are not common. Granted, just about everyone has/had an Oscar, but it's just about the only fish I really enjoy for more than looks.
I have an entire colony of Burundi Frontosa, a malawi tank, and a mixed tank. The Frontosa are my pride and joy, but none have a personality like my Oscar.