I had a very similar thread here
I have to say I'm leaning towards the Shield - just far too many reports around that the Nexus 9 is fantastic in benchmarks but far less appealing in real world usage with some horrendous tales of lag!
Personally though I like the aspect ratio on the Shield as 90% of my use is for watching videos streamed off my NAS! (well that and the inclusion of a micro SD card slot!)
Rumours of an X1 Shield coming soon if you're not in a hurry

Neither, get the 10.2" Google Pixel C 64GB instead.
Should be available within the next week or so, uses nvidia X1.
Had never heard of this and it looks nice - mind you at around 400 notes it would want to be!
Looking at the specs it's worth every penny and from Google you can expect quick updates.
The new shield tablet with X1 looks okay but I don't want another 8" tablet, seems small now.
Also the screen ratio of the Pixel C is completely difference, it's 3:2 which I think will work quite well for most uses.
I cracked and my Shield K1 is arriving today - watched some YouTube reviews and some of the game streaming looks like it could be good!
Yes I propably agree 8 inches is a bit small. When is it due for release?
The Pixel C is a completely different device to the Shield and Nexus, and cost double. So i find it strange that it's being recommended in place of either of these to be honest.
Why not? it's using the newer Tegra X1 SoC so performance should be better.
Larger display, higher resolution, brighter, better battery life, it's a great Android tablet.
Not sure about productivity use, don't really care about that.
My Shield Tablet 32GB LTE will be on only ebay shortly, you might want to keep an eye out for it.
I'm not saying it's not a better more powerful tablet. But if someone is looking at devices around £150-£200, it seems strange that people are pointing them in to the direction of a £400 tablet. Especially has both the cheaper tablets are more than suitable for the stated usage. I just don't see the merit in spending double on something that doesn't really offer much more other than spec sheet bragging and an inch in screen size.
I'm sure the tablet itself will be worth the money in the long run, it's more than powerful enough to last a number of years and hopefully Google will keep it upto date for a few years at least.
