As with most things, that's going to depend quite a bit on what you intend on needing it to beat.
The difference between 252 and 200 speed means that Cube ends up beating both forms of Gyarados, M-Altaria, Dragonite, Gallade, Mamoswine, Togekiss, Diggersby and Heatran, was well as neutral M-Garchomp Lucario, and Lando-T. Granted, the sub helps to mitigate a lot of that loss, your ability to do damage becomes a lot cleaner with the added speed. The loss in damage between 200 Attack against 252 attack is only about 4%, but I'm not sure what kills that ends up securing, since I don't really use Cube much myself, and typically just try to prepare for the worst he can bring.
Essentially, if those specific threats look like they might be enough to be worth stopping, especially those requiring a 2HKO to get the job done, I'd say you'd best go with speed. If you have enough countermeasures in your team otherwise, or don't mind losing a sub to cut these guys out after they switch, 200 speed will do just fine.