GTX 780ti SLI because performance drops hard after 2 cards not to mention the limited VRAM the 690 got. There is little reason to buy a gtx 690 after titan came out.
I'm pretty sure any kepler card does gsync. If your looking to buy I would wait a bit. Nvidia about to release new Maxwell cards soon. Supposed to be a big reveal on 19 September.
Neither. Radeon R9 295x2 or 290x Crossfire. If you're getting 780ti SLI I assume you're gonna either be playing at 1440p, 4K, or 5670x1080.4gb VRAM will probably come in handy for those kinds of things