Somewhat subjective feature and performance wise if you'll notice the difference. Some will notice it more than others - from the regular comments on this I'm fairly sure a lot of people haven't spent much time using both side by side.
Amongst other things the G-Sync FPGA has faster low frame rate recovery which reduces input lag in situations where frame rates are varied, variable [dynamic] overdrive to improve motion clarity (though that can come with trade-offs) and has better support for applications which don't use exclusive fullscreen mode (though MS keeps braking compatibility/causing issues there). As a generalisation monitors using the FPGA have a better refresh range.
Some implementations of adaptive sync (FreeSync/G-Sync compatible) are better than others - with my Philips 436M6 I'm largely pretty happy with it even with the relatively narrow VRR refresh range but notice the difference compared to my Dell S2716DG (even when eliminating the refresh range difference). I've used other FreeSync/G-Sync compatible monitors where it provides nothing like the experience of the FPGA variant of G-Sync.
As an aside despite some things which could have been better I'm quite surprised by the Philips 436M6 for gaming - with the right settings despite only being 60Hz it can provide a fairly decent experience with the combination of not having the normal VA like motion issues, fairly consistent, reasonably low, latency both input and pixel response and adaptive sync. Most monitors fall down on at least one aspect somewhere and then you feel all the inadequacies.
