Depends what your requirements are - there are still some games that use borderless window mode for fullscreen that don't work well with FreeSync and you have to pay a bit of attention to the adaptive sync range on many FreeSync monitors to get something that will work well within the kind of framerate range you'll be working with.
G-Sync has better low framerate handling but that isn't a big deal if your setup is capable of decent framerates but might be something you may or may not be sensitive to when the framerate is marginal i.e. if you are trying to push ultra/4K where there just might not be enough GPU grunt to keep things in the ideal spot.
G-Sync also has better control over overdrive for motion clarity than current FreeSync monitors but a lot of people don't seem particularly sensitive to the difference and in some cases it can be a negative - for instance in some cases you will see increased screen door type artefacts in areas of bright colour in motion, etc.
