It's always better to SLI or Crossfire if you can. I have a few reasons why I believe this to be so:
1.) You still leverage the power of the GPU you already own, giving you a much better upgrade for usually much less money.
2.) Your card design will be much more mature and stable than a new one.
3.) You spent all that extra $$$ on the beefy power supply and multi-GPU motherboard. This justifies that.