Titan X has highest core count and 12GB of RAM, double that of the 980Ti. However, a 980Ti is only about 5% slower at stock, and easily matches and exceeds (stock) Titan X performance with a mild overclock, all while costing ~$350 less. If you plan on getting the highest FPS you can in games and are ok with reducing IQ settings like AA/post processing/etc to cut down on VRAM and GPU load, you'll want to go for 980Ti's as you probably won't saturate the 6GB capacity with most games, even at 4K. If you simply plan on going for the absolute highest IQ settings (that are still playable), you'll probably want Titans as there are a few games out already that can reach and even breach 6GB of VRAM with high AA, highest texture settings, etc, when playing at UHD and Surround resolutions.
Seeing as how you mentioned wanting to game at 60FPS, you'll probably want to go with 980Ti's.