the corner tank will give it more swimming room but less distance swimming room.
more surface area.
for a foot long fish id be looking at longer tank then taller. The LFS by me that has 12" O's has them in a 1k gallon pond and they do just fine(indoor pond) but a foot long fish is huge man. best thing to do is take a tape measure, set it at exactly the length of the fish(measure him too, you can lip him just be careful, its jaw is as fragile as a bluegills not as strong as a bass) then test his length on the tape inside the tank he is giving you. go at different angles and such.
decor is gonna be a minimum, and I dont think you will have any other fish in the tank with him.
is it a pentagon corner or a euro corner tank? euro would be much much better because the angles are curves and add a little more room inside then a angles corner hex/pentagon tank does.