It's hard to describe but I think if you see each then it's hard to mistake them for each other.
The main thing with gigantea's is that they have coloured verrucae (the raised dots on the sides that they use to help adhere in place). Here's an example..
With haddoni these are smaller, more regular (like a grid), and the same colour. They're almost invisible.
Of course the tentacle length is longer on gigantea but they don't always have them all out after shipping. For example, here's a picture of my green when I first got him:
.. then here's a picture of him with longer tentacles:
Haddoni's tend to lay flatter and get bigger. Haddoni's come in a larger variety of colours though. For example you can find a red haddoni but I don't think there's such thing as a red gigantea. At least I haven't seen or heard of any so far..