If you are on G-Sync and do not intend to upgrade your monitor in a year or so, then definitely number 2. If not and you can hold out a bit longer, then hold out until 7nm from either AMD or Nvidia.
Hindsight I also regret not buying the 1080Ti before the mining craze. I game at 4K myself and just went for a Titan XP from the members market in the end. Never had a Titan before and having 12gb of vram also was enticing for playing Final Fantasy 15 which seems to be a vram hog. At least I know I am now fully covered until 7nm comes out. I really hope Nvidia actually bring the performance with 7nm and not just another 20-30%.
I personally would not bother with SLI. If you must buy today then buy one 1080ti and put the money saved towards a 7nm GPU.
They are very expensive there from what I have seen. People wanting on average £550 for their second hand gear, might as well add £30 more and get the Gigabyte Gibbo has up for pre-order brand new!
This is what I find confusing. Did the 1080 ti price went up on the second hand market as the 2080 is meh? I mean yeah I could save around £80 ish if I buy second hand but warranty is not going to be easy but then again my current cards were both bought used and I didnt have any problems with them (Maybe I am lucky?).

