If there is one thing you can count on it is an oscar getting big and getting big fast. I think an Oscar is a better pick for a large community tank, for a solo fish I would rather pick a species that really can't have tank mates. I could be wrong, but I think an Oscar ends up weighing a lot more than a flower horn so even if the fish are the same length the oscar needs more space to be healthy.
So if I had to pick I would go flowerhorn. I have seen so few 12 inch males, 9 or 10 inches seems to be closer to the norm. Regardless it seems they hit 7 inches fast then slow down from there. I would start it in a 75 and if you think you have a 12 incher on your hands upgrade him, it depends a lot on the genetics of the fish.
Besides it is a lot easier to talk your family into a bigger tank when the fish everyone loves is who needs it.