If you have any interest in PC gaming, you've likely heard about two competing technologies by NVIDIA and AMD called G-Sync and FreeSync respectively. Both are designed to eliminate screen tearing, which happens when your monitor's refresh rate can't keep up with the frames being pumped out by your video card. If you've been looking for a definitive comparison, look no further than this opus from Battle(non)sense.
The almost 19-minute video is incredibly comprehensive, covering everything from the breaking changes introduced by the Windows 10 Creators Update, through to measuring input lag caused by both technologies, as well as regular vertical synchronisation.
If you just want to see FreeSync and G-Sync duking it out, skip to the 11:23-mark, otherwise, I highly recommending watching the clip in full for a complete background on the advantages and disadvantages of each vendor's approach.
For those after the bottom line — they both get the job done. In terms of which is better, it appears that would be G-Sync, however, compatible monitors are more expensive. FreeSync options are cheaper, but you'll have to spend a bit more time fiddling with frame rate limiters to get it working as well.