crossfire has better scaling atm, furyx crossfire pretty much matches up, beats 980ti sli in some games, loses in others,
but the custom 980ti's you are talking about all run for $700+, you're talking about a lot more money, seahawk goes for $750, lightning for more. If you're going to make comparisons you should really be talking about the baseline 980ti models, ie MSI basic, evga basic. These "custom models" you refer to will cost OP at LEAST $150 more combined than basic r9 fury x models. The sapphire radeon fury x is currently running for $610 on newegg, and that model is liquid cooled.
Plus, on top of all of that, it's good to support the underdog in the battle for graphics supremacy, as AMD is kind of banking on support due to their lack of new CPU's in the recent years. This isn't to say that they are making worse cards, infact their most recent line up of r9 cards are quite fantastic.
And i'm not an AMD fanboy, i currently run a gtx 970 in my system