I'm not sure how the warranties work where you are located. I am in the US - most of the major brands offer 3 year warranties, but do NOT allow warranty transfers. EVGA has some allowance for warranty transfer.
I don't know what country you're in, or how exactly your consumer laws affect this.
With your current monitor, the 1080 would be a waste. It has more performance capability than your monitor can make use of.
If you move to a 2k ultrawide at some point in the future, if your max refresh rate is similar to what you have now, then even the 1660Ti will still be able to max it out in most games most of the time.
I guess it also depends on how long before you plan to upgrade the monitor - if it's a few years down the road, then I would definitely stick with the 1660Ti. If it's going to be sooner than that, then my decision would be based on:
1 - guaranteeing for certain that the warranty transfers to the new owner, and knowing exactly how long that will be, if you get the 1080
but more importantly
2 - What resolution (2560x1080 or 2560x1440) and refresh rate will your new monitor be.
EDIT: The conundrum of PC upgrades - each time something is changed, other decisions have to be recalculated, and more questions need to get asked.