I think it depends what you want and 144Hz will give you the speed but 4K gives you fantastic details of the scene. Everything is just so beautiful, fine and effects are amazing G Sync or Free Sync helps you get rid of the lag whenever possible.
There are already Free Sync LCD's and they are cheaper than G Sync BUT it does not work on Crossfire and have slightly more problems than G Sync. You can google and read about them. The only disadvantage is that these adaptive refresh rate is still very proprietary towards specific GPU brands. It means you cannot just swap from ATI to NVidia once you decided on the LCD.

To me, it will always be NVidia so no issues in having a G Sync monitor. Once you've tried it, there's really no point getting those normal LCD's anymore for gaming.