While the Shield looks like a good tablet I would not buy it personally based on the history of Nvidia based Android hardware. In almost all/if not every case where Nvidia brought out a "killer" SOC it is under powered, unoptimised and outstripped by the main competitors in a matter of months and usually more expensive in the beginning. This is leaving out Nvidia's complete bastardisation of all things Open Source and linux, which Android is built from. While the stats seems to suggest this will not happen this time with the K1 I do not put any stock in rumors and certainly not in preview write ups.
As for the "superior tablet in several ways." I have some thoughts on that.
The screen is larger but the Nexus 7 is the perfect size and weight for one handed use and comfort, the shield will be bigger and heavier so may not be a great carry around every day use device but better as a home based gaming and media consumption device. Personal opinion: Nexus 7 wins for usability but we will see in the real world when the Shield comes out.
The screen being larger also beings the problem of lower PPI (Pixels Per Inch) which will make the display marginally worse to look at, pair this up with the fact that the Sheild only has a 1080p screen compared to the Nexus 7's 1920x1200 screen the quality seems oddly low. Not bad at all but not what I would call Superior, especially to showcase the raw power of the K1 and its gaming prowess. Seems to me to say that the K1 can't drive a high density screen and therefore they are making the parts fit the K1 rather than having K1 meet newer part quality standards, which are getting higher with each new device.
Tablet and phone audio is always bad regardless of how good the implementation and 90% of the time you will likely be using headphones anyway to not annoy others around you so that point is moot.
The SD card is a legitimate point/concern. Although I would be happy with a 32GB Nexus as I use my phone for music and that is where I need the SD card. A tablet for me would be for gaming and that is just personal, I would not need an SD card and even then it would be useless as Android have instated that you cannot move apps to the SD any more as part of Project Butter. So for games which the tablet is aimed at the SD slot is useless.
A dedicated HDMI port is not only unnecessary but also setting back the standard. MHL has been able to do this for a long time. Why even add a HDMI slot? I might be missing something?
The Stylus is a very nice feature. I use one every day with my Note II. So that is, for me at least, a huge plus.
Even though you did not bring it up I must add that the controller is not a selling point. Android can work with a DS3 controller, Moga controllers and many others and they are not Proprietary and tied to only the Shield for that point alone I see it as a mark against the Shield. Proprietary hardware needs to go away. If I am wrong, I will change this point but it seems to be the way with Nvidia tech. And as far as I know the Shield does not even come with one so more expense for a "killer" feature.
The other "killer" feature is the streaming which requires an Nvidia GPU and you said you have an R9 280x so that does not even factor in. There is word that Valve might be making a generic streaming based around the Steam In-Home Streaming that will work on any android device with any GPU. As far as I know this is not 100% confirmed but might be worth looking into as it would potentially mean devices like the Nexus 7 would be able to Stream PC games as well.
Gaming tablets come in two flavours for me. The Windows ones will give you traditional PC gaming but will fail hard because the GPUs inside have to be nerfed due to heat and battery concerns, so they are only good for low level indie games or old games. Android can do what looks and feels like modern games and more importantly different games. The games on android are vastly more causal, quick and in a lot of cases interesting. They make use of accelerometers, touch, controller, text input in some cases, camera, augmented reality and probably a few more to come, so they definitely to me seem the better platform for on the move gaming. Android can also run emulators for everything up to Dreamcast and Wii, although not very well in the Wii's case, so console and old games are covered by android as well.
For me the Nexus 7, right here and now, wins outright. This might change with the official release of the Shield but I suspect the next Nexus or other wise high end tablet will be better than the Shield anyway. It is all in the future at the moment, but like I said, right here and now for me personally is has to be the Nexus 7.
Also The LG G-Pad seems a really good alternative to the Nexus.