Of couse 1440p is a great investment! Otherwise, it looks like you're buying a $700-800 graphics card for the <$100 monitor. The GTX1070 is considered a 1440p card, then there is GTX1080, which is even better and then there is GTX1080Ti you're buying, which is noticeably stronger than the GTX1080! of course go for a better display, because:
1. That GPU (1080Ti) is an insane overkill for 1080p.
2. It would simply be a shame to use GTX1080Ti on 1080p monitor, except 1080p@240Hz gaming cases.
3. That card is great even for 4k and the 1440p is 2k, so you can expect great framerates and ultra details at 1440p resolution for a few years for sure, it will be a very future-proof setup if your other components can handle the 1080Ti.