First of all, I'd like to say that's going to be one killer rig. I'm jealous of you... Haha anyway everyone above is correct. If you're gaming at 1080p, its not worth going SLI or Crossfire immediately. I'd recommend getting just one card and either one of those would cards you listed would be considered overkill right now for 1080p. If you really do plan on going 4k in the future (or 3 1080p monitors for surround/eyefinity) I'd pick up the second card before you switch resolutions. Though for either high res setups, the amount of vram may become an issue... That's where the Titan Black would be a better option if you really do plan on going to a higher res. If you aren't sure about going to a higher res, I'd recommend staying away from the Titan and getting the 780ti or 290x.
Now for crossfire or SLI. I only have personal experience from my own SLI setup, and from it I can say I've had no problems with SLI. Nvidia is good with releasing quality drivers quickly. I've heard the AMD doesn't produce drivers as well as Nvidia, but I have no personal experience with AMD as I am a loyal customer of Nvidia
and cannot verify the truth of that. Either route you go will result in a mouth watering rig.
Now if I could rep Nvidia a bit, I'd maybe point you a bit towards running sli 780tis as the 780ti has quite a bit of oc headroom. I've read of 780tix hitting 1200mhz consistently leading to a increase of performance by another 10% while maintaining cool temps. The 780 ti is more power efficient and stays cooler, while being able to oc quiet high and so I'd personally go for sli 780tis over crossfire 290xs.