Here is the spec comparison sheet:
The most obvious difference is the screen size. The Nexus 9 has an 8.9" screen while the SHIELD Tablet has an 8.0" screen. Although the resolution is higher on the Nexus 9, the ppi (pixel per inch) is greater on the SHIELD Tablet so theoretically it should be sharper. However one has to physically compare the screens to determine how they look. The aspect ratio is also very different. Google went with a 4:3 ratio similar to Apple's iPad while NVIDIA went with the standard 16:9. Videos & movies look better on 16:9 screen while browers and certain apps look better on 4:3 screen. This would be up to the individual preference.
The Nexus 9 is longer, wider and thinner than the SHIELD. However, the SHIELD Tablet is lighter (390g vs. 436g).
Picture courtesy of Google.
The internal hardware is similar with the biggest difference being the CPU. The GPU is the same NVIDIA 192 core Kepler 1 SMX so again theoretically the gaming experience should be similar but they will be different because of the software and CPU. 3D games and other high performance games primarily rely on the GPU but also the CPU to a certain extent. In addition, NVIDIA is currently running a slightly modified Android 4.4 (but 5.0 will be available later this month) while Nexus 9 is running unmodified Android 5.0. The Nexus 9 uses NVIDIA's first 64 bit processor - Tegra K1 Dual Core 2.3GHz while SHIELD uses 32 bit Tegra K1 Quad Core 2.2GHz. Don't let the lower processor speed fool you into thinking its slower. Until benchmarks are performed on both, it is hard to tell which is "faster". Also at the moment, very few, if any, apps take advantage of the 64 bit processor. Both have 2GB of RAM.
Other notable differences: SHIELD has expandable storage while Nexus 9 does not (huge difference); the front/rear camera resolutions (1.8MP/8MP Nexus; 5MP/5MP SHIELD); SHIELD has a stylus while Nexus 9 does not; SHIELD includes mini-HDMI port while Nexus 9 does not.
Pricing is also different. The Nexus 9 is more expensive - $399 for 16GB WiFi vs. SHIELD's pricing of $299 for 16GB WiFi while the Nexus 9 32GB WiFi-only will cost you $479 compare to $399 for the 32GB WiFi SHIELD which also includes LTE.
Overall on a spec comparison basis, the NVIDIA SHIELD Tablet seems like the better overall value. However performance is what counts. For that hands-on testing would provide that. We will try to obtain a unit to perform a full review. Stay tune.
Picture courtesy of NVIDIA.