An ocellaris is more hardy. The above statement about dorsal spines is true...don't remember if the numbers he gave are exact, but you can do a google search to figure that out.
There are many differences in the two that were not mentioned.
1) Location. they do not come from the same waters...in fact, their localities do no even overlap.
2) Breeding. Oc's breed pretty much on the same cycle as perculas, but that is where the similarity stops. Their hatching is usually faster than perculas, their period to MMP is much faster than percula, and their patterning and colors form much faster than percula. In fact, many percs will take over a year to gain adult patterns, whereas many oc's will be ready for sale at 6 months.
3) Color. This is a generality and is many times not a standard, just good reference. Most oc's will have a stable color by 6 months(there are exceptions like the B&W's). their orange will be a lighter color and the space between the orange and the white will usually have a very thin line of black that will never grow as they age. Percula's will have the black usually grow with their bodies. They are usually a darker, and to me more appealling, color orange.
4) Hardiness. This has been discussed, but I'd like to add. Yes, oc's are considered a more hardy fish, but I think this is a little bit false. I think this comes from breeders who were breeding both percs and ocs and saw the quick MMP, the higher ratio of viable offspring because of the quick larval period, less cull count due to deformities, misbarring, etc. I think that when the fish are the same full juvi size and when tank bred(outside factors of disease, transport, stress, etc will alter these ideas completely) they are both as hardy as one another. if you care for your fish and don't put it through a whole lot of stress, percs should do absolutely fine in your tank.
