If the sentience of your food enters into your ethical calculation, then there is a huge difference between oysters--almost plant-like, and octopuses, which are highly intelligent. Oyster farming has a minimal effect on the environment, and oysters gathered from the wild are pretty eco-friendly too. Octopus "fisheries" in California and Hawaii are also considered to be
well-managed, so from a sustainability perspective, they are both good choices.
Well, you hit on two issues here:
1. If the sentience of a creature (it's state of conciousness or sensory awareness) should determine whether or not you'll eat it
2. The sustainability of octopus fisheries, and the impact using them as food has on the environment
My response to #1 is: I suppose spending summers on my grandparents' farm in Tennessee innoculated me to feeling squeamish about eating animals - even those that display some degree of sentience. When you've spent half the summar lovingly caring for an adorable calf with big brown eyes, and then you bring him to market with your grandfather and come home with sausage, and later help your grandma cut the heads off chickens and pluck them for dinner, that kind of takes the mystery out of where your food comes from. Unlike so many others in our restaurant-and-supermarket-fed society, I don't suffer from the illusion that all meat originates in cellophane wrappers. I'm well aware that much of the protein I eat used to be quite alive, and might even have had feelings. That's just the way it is on this planet - some creatures eat others. And I have no problem being at the top of the food chain.
However, I also recognize the importance of ecological balance, and strongly believe in choosing what creatures I will eat based on whether or not eating them will cause an imbalance. I support avoiding seafood that has been reported as being at-risk for overfishing, and I make a point of keeping up with seafood sustainabiilty lists. I know to avoid chilean seabass and bluefin tuna (severely overfished), and to choose salmon that is Alaskan wild-caught, not farmed (not only is it not overfished, but it's better for you anyway!).
Hence my response for #2: fortunately, as you point out, octopus as a species are not considered endangered. Octopus are used for food in many parts of the world, and while there are some concerns about overfishing in some areas, these concerns are no more or less than they are about almost any other species of fish that are consumed by humans. So as an eco-friendly type of seafood, they pass the test.
That still doesn't make me want to eat a live baby octopus! But I have no problem with those who do.