By nature, Ocellaris in pairs tend to be a little more aggressive and have a larger territorial area. Percula's are not as territorial. As far as care level, they're the same and both reach about the same size. There is very little difference between Ocellaris and Perculas, so in all honesty, I'd go with what you like the look of better, just know that paired ocellaris, especially if they ever mate do tend to be more aggressive.
I've always kept ocellaris and ocellaris morphs and really never had an issue, even when they were breeding. The only time they every attacked another fish was when my flame angel tried to eat the eggs.
A maroon clown would be a terrible choice for this size setup.