A 4k panel with gsync is really nice. It will keep your gameplay very smooth even if you dip below 60fps. The 1080ti can keep 60fps on most games. But there are some to keep that high framerate you will need to drop settings. Monster Hunter World is fairly demanding at 4k. But it is not the most optimized port either.
But if you play competitive quick twitch shooters, then you will like the 144hz refresh rate. It does make a difference. AC Origins looks great at 4k and wont see as much benefit with higher hz as say BF1. 1440p looks really good too.
Have you considered an 3440x1440? It is sometimes called 2.5k. You can get them up to 120hz and they are like have two 27" monitors together with no bezel. Most games support...