The difference between what two? I'm guessing you mean the difference between a true percula and a false percula (a.k.a. ocellaris). They're both clownfish, and they're very, very similar. Both are pretty hardy, and I don't know if I'd say one is more hardy than the other. As a matter of fact, there are quite a few marine biologists who believe they may really be the same species. True perculas are usually your more expensive fish, though. Most fish stores will carry both, but ocellaris are definitely the more common of the two species, so you'll see them in stores much more often.