After testing the two cards in another computer. The 512mb version seems to be much more responsive than the 1gb.
However, I did manage to get the 1gb version clocks to match that of the 512mb and it was stable two.
When it comes down to it, it's how much you want to spend. Hope this helps
Crysis Benchmark:
1GB = VeryHighSpec, OC, 36 - 40 fps
512MB = VeryHighSpec, 34 - 41