Not all games support SLI/XFire and it could suffer from usual SLI/XFire issues e.g. micro stutters.
As long as you can find a single GPU which can outperform or perform near 2 cards on SLI/XFire, it is better to pick that single GPU, unless you have already one of the needed GPUs.
For now, a single GTX980Ti or Fury X is you best bet.
If you really need more power, XFiring/SLI-ing GTX980Tis or Fury Xs is then good.