I personally would go with the 2x 970 because when it comes to SLI you want to do two things
1-Get the largest single card you can get before trying to run SLI
2-Have large amounts of Vram to be able to run higher resolutions.
The 970 on its own is a much stronger card than the 280x, and it has an extra gb of vram. Also the 970 will draw far less power and, and most likely run quieter.