Cutting the memory on the video card would affect how much you can load on the video card. The GPU is still the same. If your renderings are large and completely detailed running all the time (like a full and detailed background, with each object in that scene individually rendered with a ridiculous amount of detail, i.e the works, 24/7), then a video card with larger memory would be better. But if you are not going for that level of detail all the time, then the card with less memory would be better.
But the BFG card is overclocked so it could handle some of the aspects of the higher memory capacity card without the price tag. BFG cards also carry a lifetime warranty on their cards.
http://www.bfgtech.com/tech_redux.html