i'd go with a single 1080 ti over sli 1080. ti is a 35% increase over 1080 and sli will only boost performance over a single 1080 by 10-15%. not only that but, the 10-15% is only in games that actually work with sli AND are optimized well for it. i'd put the extra money in a better/bigger ssd, better ram, or just set it aside for a second ti. currently doing a build with a 1080ti and did a fair amount of research on the sli vs ti.