From my single octo (pygmy - Mercatoris) experience and reading the forum (Anecdotal evidence only), another difference I have observed is food choice. My Mercatoris female and now her 5 babies totally ignore snails (except to use their shells as door ways), baby fish (possibly too fast) and hermit crabs (I will try the babies again with these soon). Both adult and babies will gladly eat Cyclop-eeze (and possibly other filter type food) and live shore shrimp. The mother, at full adult, would eat fiddler crabs IF they were presented to her (I don't believe she ever ate any loose in the tank) and she MAY have eaten one mithrax crab (not presented but just living in the tank). The larger octos seem more willing to hunt or accept larger shrimp, some fish as well as snails and hermits.
It might be good to start a dedicated thread, "what my octopus eats" and qualifying the entries by the size and type of octopus.