As an owner of both green & red rigs I'm looking forward to Mantle more, purely because I get to see it's benefits without spending any more cash. It's also going to be a good few years before G-Sync (if it takes off) gets to a saturation point where you are not limited to choice of monitor and is (for now) off limits to multi monitor setups. I would also be loathed to spend extra money on what is arguably the least often upgraded component only to have that extra feature redundant should I plug it into an AMD based rig.
As I game at 21:9, I'm far more interested in bigger 21:9 screens than I am G-Sync enabled screens. Obviously a bigger 21:9 G-Sync screen would tick all the boxes though - at the right price
Lastly, G-Sync does have it's quirks and limitations. As Anand pointed out, it seems to work best between certain frame rates and is not perfect even then. Mantle will have no such quirks, it will just be faster period. The only drawbacks are going to be in people's expectations rather than of the system itself.
As I game at 21:9, I'm far more interested in bigger 21:9 screens than I am G-Sync enabled screens. Obviously a bigger 21:9 G-Sync screen would tick all the boxes though - at the right price
Lastly, G-Sync does have it's quirks and limitations. As Anand pointed out, it seems to work best between certain frame rates and is not perfect even then. Mantle will have no such quirks, it will just be faster period. The only drawbacks are going to be in people's expectations rather than of the system itself.