a 5960k w/ 2 GFX cards will be matched by a Z170 w/ 2 GFX cards as a gaming box. All X99 really brings to the table is support for 3 and 4 GFX cards and the extra scaling you get from that can range from negative to significant but whether it's worth it will depend on the individual. "Better" is a relative term and 3 way is certainly better than 2 way from a performance only .... tho from a performance per dollar standpoint, 2 way holds the edge there. Of course w/o telling us your resolution, it makes answering your question difficult an answer varies based upon resoluition
http://www.maximumpc.com/geforce-gtx-980-ti-3-way-sli-overkilling-it/
At 1440p.....
Metro LL goes from 106.3 fps w/ 2 cards to 105.5 w/ 3
SoM goes from 138 w/ 2 to 141 w/3
TombRaider goes from 173.1 w/ 2 cards to 210.3 w/ 3
Witcher 3 goes from 71.2 w/ 2 cards to 82.6 w/3
At 2160p.....
Metro LL goes from 64.6 fps w/ 2 cards to 64.7 w/ 3
SoM goes from 89.5 w/ 2 to 111.2 w/3
TombRaider goes from 86.5 w/ 2 cards to 104.2 w/ 3
Witcher 3 goes from 41.7 w/ 2 cards to 46.9 w/3
I would recommend Z170 w. 2x 980 Ti unless either....
1. You are doing rendering or other workstation apps that would benefit from X99 platform, or....
2. The extra $1500 means nothing to you.