I have a friend with a 6950 flashed to a 6970 however these cards are becoming harder to find. If you can find one however I highly suggest going that route. If this is not the case then I am going to have to disagree with the above posters.
I personally would go with the CF or SLI option. Unless your running your games at a insane resolution. As for games having problems with such this isn't really that common today and is normally fixed through driver updates anyways. The judgement between these two should go by what prices you can currently snatch them for. If by any chance you are able to get them for a price differnce so minimal than ask yourself if Physx is important to you and decide through that.