bjornl :
geofelt :
Freesync applies to amd cards; G.sync applies to nvidia cards.
Either option is designed to let the monitor pace the graphics card.
Usually, freesync monitors are cheaper.
I think, buy the monitor that pleases you with the image quality, price, and resolution you want.
A GTX1070 is likely to drive anything to sufficiently high FPS for anything except fast action games.
For those, a G.sync option might be better.
Google fast-sync on free-sync monitors and you will find the flaw with your post.
Not disagreeing that g-sync is better. But it is not my money so I can't decide for the OP if it is better-enough to warrant the ~$200 surcharge.
Good suggestion, being no expert I did.
My reading says that fast-sync is applies when the graphics card can output frames faster than the monitor can handle them. Sounds like a good idea.
However, in the OP case, it seems likely that the proposed GTX1070ti may be the limiting factor with a 4k monitor.