From my experience, the aqua-scaping that is done in the tank will matter more to the octopus than the dimensions of the tank itself. I would recommend something that best fits the space you have in the room you plan to put the tank in. All the suggestions you made would work, depending on how you structure the in-tank environment. All my octos have always been in rectangular tanks and they have done just fine. The volume of the tank matters more than the specific dimensions, as long as you aren't planning anything crazy, like a 6" wide super long tank or something else wildly different than standard tank specs.
A small side note- if you have a tank that is not as tall but really long and wide, you may have more trouble keeping the lid secured and octo-proofed as compared to a tank that has a smaller footprint.