people have been having this argument for decades; 256mb vs 512mb, 512mb vs 1GB, 2GB, vs 4GB, etc., etc.,
reality is when a card with the exact same GPU comes in two memory offerings it's often too slow to use the larger one to it's full advantage. So regardless of the version of the GPU you get, you are often both lowering settings or like many other people moving on to better performing cards.
As we stretch into scenarios of general unplayability on each device, e.g. 26FPS AVG vs. 30FPS AVG, we do start to see differences which are “significant” from a pure percentages standpoint. This makes sense, as the numbers are becoming smaller and so the delta looks wider, but the reality is that a player would likely be equally unhappy at 26FPS and 30FPS. For differences to really start showing large percent deltas between 2GB and 4GB cards (in our tested games), we have to push settings to a point where neither device would be particularly good to use, giving us an academic exercise rather than a practical comparison.