Differences between AMD FreeSync and Nvidia G-Sync
Essentially meant to stop seeing screen tearing Input lag and stuttering.
It's not one is better than the other rather they work together.
Nvidia was first on to the market with its G-Sync, The technology is impressive but it has a slight drawback. In order to be G-Sync compatible, the screen needs G-Sync specific hardware that's rather expensive, depending on the spec of the monitor.
One clear difference between Nvidia G-Sync and AMD FreeSync is how graphic cards are handled.
Cards that produce higher frame rates than a monitor can handle. G-Sync locks frame rates to the upper limit of the monitor while FreeSync (with in-game Vsync turned off) will allow the graphics card to produce a higher frame rate. (this can introduce tearing), but also means that input lag is at an absolute minimum, which is important for gamers such as those who play FPS titles.
Courtesy: AMD. Hope this helps.