Why would you want to do SLI in the first place? A 1080 Ti is more than capable of delivering Ultra quality over 60FPS at 4k, and well over 120 at 1440p, there isn't anything even out that you would need to push it more than that right now. And by the time that becomes a reality something far better will be out in the world of GPUs. The difference between 100fps and 60fps is smaller than the difference between 60 and 50fps so adding another card and pushing 200fps on most games is really pointless since you can't even tell the difference.
Despite that, 2.5 slot cards are horrible for SLI, the top card is going to be significantly hotter, always, as it is pretty much always choked for air. And you have a Formula so you will have to use the 1 and 2 slots. If you are going to do SLI I would do an MSI Sea Hawk EK X. An EK loop is expensive as hell, but it's the ideal cooling solution as it is quiet and incomparably cool and if you want two cards, there's no better solution.
Having said that. Between those two strictly I would choose the FTW3, I have looked at it and the only reason it's not an option for me is because I have a personal BIAS against EVGA products, but even I have to admit it looks BA and it could easily be the best 1080Ti available because of the dual-BIOS. But I have never even heard of Auroras so I have no idea what they offer.
For me the challenge is choosing between the Asus ROG Strix and the Zotac AMP! Extreme. I love my AMP! Extreme now, but Asus managed to compete with a 2.5 slot card as a 2-slot card in the 1080 category, if they improved their cooling as much as it looks like they did, it will be no comparison and the Asus ROG Strix will most likely be the coolest, quietest, air-cooled cards with a top-tier OC. Zotac has been the fastest but they are hot and loud because of the Overclocking done to them, if I can get within the same OC range within 1-2 fps like the 1080 version, but significantly cooler and quieter, i'm going with the Asus.