either is good. the 6950 is slightly faster at stock but the 560ti is better for OC. both cards are about the same speed once you OC both. the 1gb 6950 is enough as no game will use more than that for 1080p gaming.
Both those cards are very good options.But since the release of Nvidia's new lineup is only a few months away and AMD is still launching the rest it's middle to upper class cards I suggest waiting a bit to see what the market does.Or maybe even get one of those cards.