The actual G-Sync tech works better, but the premium you have to pay for it is ridiculously high.
Honestly though, choosing between the two right now depends on your budget.
/thread
I have my brand preference for AMD but at the end of the day whichever card is faster for your price point is what you should buy unless you have other considerations that are brand dependent like PhysX or Cuda.
Driver support from both brands is all in all good at this point. AMD is notorious for bad drivers but those days have passed, at least to the point where they're not noticeably worse than Nvidia's. If anything I'd say AMD has better driver support (recently) as they support their products for longer and performance on their cards can drastically increase over time. Certain cards like the 7970 have gone from competing with the GTX 670 at launch to matching a GTX 780 on newer titles. This obviously varies depending on the game and card but generally speaking you'll get more longevity out of an AMD card.
On a somewhat more business level I generally dislike Nvidia. Gameworks includes licensed technologies sold to game developers that run noticeably worse on AMD cards. On the flipside AMD has custom tech too (like TressFX) but release them for free to anybody and work just as well on Nvidia cards. This obviously is better for everyone but as it stands now their techs aren't quite as advanced as Nvidia. G-Sync also is way more expensive than Freesync and all in all I feel AMD is more consumer friendly.
You can argue till the sun comes up about it but really it's just what's best for your money. I don't see myself buying an Nvidia card anytime soon, but if they offer a better card for the price when I need an upgrade then I'll buy the better card.