NOrmally, I would say - go for the "standard" tank. They generally tend to have more surface area than the "cube" tanks. But these are both the same depth - and the cube has about 420 square inches surface area compared to approx. 360 square inches on the standard tank. So the cube is better in the surface area department.
Ranchus can get big and are powerful swimmers, but having 20 inches to swim PLUS 21 inches to turn in is GREAT. I have always had a problem with the narrow, 12 inch wide tanks with big fish. It bothers me to see the tail on one side and the head on the other when they turn. So the wider tank is better in my opinon.
You can keep 2 large Ranchu or 3 medium large Ranchu in the 30(29) gallon tank with only the standard amount of work. As your fish grow, you will need to adjust the amount of water you change out at each cleaning and, probably, shorten the time between changes to accomodate the larger fish/larger waste.
I am not familiar with the Fluval 404, but from what I figure that is 404gph, right? That is NOT too much for the 30 gallon tank. I prefer 12 to 15 times an hour turnover anyway. Ranchu are powerful fish. They will do fine with that.
That only leaves the fact that sometimes extra or replacement parts - such as a hood - might be a bit more difficult to get, but... they are available if you look.
Get the cube if you can swing it. It sounds very attractive.