Bimaculoides are a good octopus to keep primarily because they live a relatively long time. However, they are a cool water species and unless you can supply temperatures under about 21C, there can be problems. Where they are found in Southern California summer temperatures run 18-20 and winter temps can get down to 15. Without a chiller, most home aquaria are running at 22-24 or higher. This can stress a bimac. We rear ours at 15-16 and they do quite well, some living over two years.
O. mercatorus do well at typical ambient temperatures and have the additional advantage of producing large eggs that hatch directly into demersal juveniles that do not pass through a planktonic paralarval stage. This means that you can rear them without too much trouble. The disadvantages are that they are nocturnal and secretive, so you won't see much of them and the life span is relatively short, usually under a year. In fact, you purchase an adult, they will usually not last more than a couple of months.
I would recommend octopus in the genus Abdopus, particularly A. aculeatus. They are readily available from several suppliers importing invertebrates from Indonesia and the Philippines, relatively inexpensive, are and extremely common shallow water species so there aren't many issues regarding conservation, they are tolerant of fluctuations in salinity and temperature, and they are day active so you will see them out and about. Large, mature A. aculeatus usually reproduce and scenesce within two or three months, but if you can find a juvenile (mantle length 2-4 cm), they can last in the aquarium for several months provided you don't over feed them.
Roy
