Before comparing the newly released version of the two decent graphic device-makers, the important thing to know is why the other one is better than the other. AMD Radeon RX 490 and NVIDIA GTX 1080 Ti are two devices that have better screen quality with high FPS which makes them far better in terms of capability and performance than all other devices offered now in the market.
Both these devices have the extreme category of GPU (graphics processing unit) with a new memory architecture (Radeon have GCN 1.3 Greenland XT while NVIDIA with Pascal P104-400-A1). Although both devices have the same performance they also have some differences.
The GTX 1080 Ti has a 557 MHz higher core clock speed than the RX 490, but the RX 490 has 96 more Texture Mapping Units than the GTX 1080. As a result, the RX 490 exhibits an 11.7 GTexel/s better Texture Fill Rate than the GTX 1080. This shows a better shader performance particularly since both of this GPUs support at least DirectX 10.
The AMD RX 490 has 716.8 GB/sec greater memory bandwidth than the GTX 1080 Ti, which means that the memory performance of the RX 490 is massively better than the GTX 1080 Ti.The Radeon RX 490 8GB has 4096 Shader Processing Units and the GTX 1080 has 2560. However, the actual shader performance of the RX 490 is 4301 and the actual shader performance of the GTX 1080 Ti is 5989.
The RX 490 transistor size technology is 2 nm (nanometers) smaller than the GeForce GTX 1080. This means that the AMD Radeon RX 490 is expected to run very slightly cooler and achieve higher clock frequencies than the GTX 1080 Ti.The Radeon RX 490 8GB requires 225 Watts to run and the GTX 1080 Ti requires 180 Watts which may have a slight adverse affect on your yearly electricity bills in comparison to the GTX 1080 Ti.
So although Radeon not has been released yet with some other data, that's why in terms of overall gaming performance, the graphical capabilities of the GeForce GTX 1080 Ti are marginally better than the Radeon RX 490.