The oscar won't outgrow the butti. The biggest buttis I've seen were about equal to big oscars. I have seen one oscar that elapsed all others but thats one of those rarities, he was 4 inches wide.
The butti will be just as if not more interested in you as the oscar. The problem is the butti will descale an oscar if put together. I would say that either would be just as interesting as the other. It would mostly come down to which you find more attractive. They are both on the top of my list of favorite fish. Enjoy whichever you get but don't put anything with a butti.
he was a good fish