I recommend the 980Ti, here's some reasons why I think so.
Less power required.
Less heat output.
SLI does not equal double the performance. (Some games can't even take advantage of SLI. AT ALL)
Ability to add a second 980Ti later.
Performance of 970 SLI and a single 980Ti is not very far apart from each other.
You can sell your 970 to help cover the costs of the 980Ti upgrade.
Oculus rift required high FPS and resolutions, something the 980Ti is quite good at. (thanks to the extra VRAM)
At the end of the day it's up to you, both methods will perform very similarly.