Generally speaking you are better off going with the more powerful single card rather than two less powerful cards. When it comes to VRAM two 1GB cards effectively only use 1GB or memory, as the same VRAM data is copied to both cards. The 6970 2GB would use the full 2GB, if you can find software that actually needs that much, or if you game across multiple monitors.
While the 6000 series does have very good crossfire scaling, and I think the 2 6870s could theoretically slightly outperform the 6970, a lot of people have been reporting driver issues, leading to microstuttering and reduced framerates in some games, particularly when crossfiring 2 6870s.