Ocellaris will pretty much always accept a gigantea as a host. It is about a 50-50 whether or not they will accept a haddoni.
Haddonis are waaaaay easier to take care of than gigantea. Haddoni are on a level with BTAs as far as care. But they are more likely to make a meal of other fish in the tank than a gigantea. Giganteas may be the most difficult host anemone to find in good condition and maintain.
On a haddoni the opening to their mouth has a thickening of the tissue that makes it look like lips. The "lips" are usually red, pink or yellowish. Giganteas have only thin tissue around the mouth, and it is the same color as the rest of the disk.
Gigantea will have blue or purple spots (verrucae) on the underside of their oral disk. These spots will sometimes fade when the anemone is in poor health. Haddoni almost never have colored spots although they do have faint raised bumps.
The column of a haddoni is almost always white or cream, sometimes a very faint pastel color. Gigantea columns are often green, blue or tan, but can be white as well.
The tips of a haddonis tentacles are usually blunt while the giganteas are pointed, but it depends on the animals health so it is not a reliable tool for distinguishing the two.
In a low flow situation the tentalces of gigantea will often move on their own while those of a haddoni usually don't, but again it is quite variable and not a reliable method of ID.