If you want the best solution, regardless of price, go with the GTX Titans in SLI. The 690 is a horrible solution for anyone playing on high resolutions because it only has 2GB of effective vram. Also, as shown previously, it is a weaker solution than the Titan in terms of raw performance. Also, it's more taxing on your power supply, has a less efficient air cooling system (resulting in crappier overclocking), does not have GPU Boost 2.0, and you're out of expansion options, considering you're already in quad SLI and with 2 Titans you can still get 2 more.
The 7990 is superior to the 690, but I really don't think you'll want to deal with Crossfire, especially 4-way Crossfire. All previous arguments hold except for performance. I have not seen much on the 7990, but just look at the Aries II and tone the benchmarks down a little to get an idea. To make things easier for you in the future, go off of the following list when comparing GPUs:
- Price per performance
- Overall performance
- Vram
- Expandability (how many cards can you run in total?)
- Power consumption
- Cooling solution (if you want to overclock, this is essential)
- Multi-GPU Scaling
- Bonuses/Gimmicks (GPU Boost 2.0 is an example)