Until the recent 5.0.1 update hit the SHIELD Tablet last week it was lagging and not very battery friendly on Android 5.0 (Lollipop). Now the update has smoothed the experience out like it was on KitKat and battery life seems to be returning.
The Nexus 9, isn't as bad in my opinion as others make it out to be. I feel any device that you sit back and play games or watch video on will take a hard hit to battery. Sure the N9 takes a long time to charge up but once it does watching my kids play YouTube videos and play some games and surf the web it holds up very well.
Still not completely answering your question, I think it comes down to personal preference.
- Are you into gaming? Then SHIELD Tablet is an easy winner
- Overall device size/screen. SHIELD Tablet is naturally smaller in over all size and 1 inch smaller display but it is also a tad heavier than the Nexus 9. Same token the Nexus 9 isn't that bulky by my standards, I find it very comfortable to sit back and use and its lighter.
- Cost.... Nexus 9 will set you back much more to buy. NVIDIA on the other hand has been running some great deals up to the holidays on the SHIELD Tablet combo controller & tablet for a great price. But aside from the recent promo's NVIDIA still costs less.
- Support... Google is good at keeping the OS updated, but its all wrapped in HTC's shell and therefore HTC has to take part in providing software improvements and updates to their own pieces of the device. This doesn't always play out very well... NVIDIA on the other hand handles the entire package on the SHIELD Tablet from front to back and inbetween. They handle the software and the hardware, its a one stop shop when something goes wrong or requires updates. And NVIDIA has a great track record going of updating their devices.
- Expandable storage... Nexus 9.. (Cloud) SHIELD Tablet (removable sdcard). Enough said if your into that sort of thing.
Those are some key factors in my opinion to look at but do your homework, watch and read as many reviews on both as you can and make your best educated purchase from there.