The Shield isn't meant to compete with anything except other portable systems, such as 3DS and Vita. OUYA and Shield are in completely different worlds. The only real common feature is that they run on Android.
One major reason for the Shield is to break "hardcore" gamers away from ATI video cards. To stream from your computer, you need a current gen NVIDIA video card (minimum 650 for desktop, and 660 for laptop). Nobody is going to buy a $400 handheld (my estimate on cost, purely speculation) and not take advantage of the ability to stream games.
Of course they're also trying to make PC gaming truly mobile. Right now the most mobile you can get is a slate PC, but then you lose the ability to play high end games. I am not about to carry my ASUS G75VW to the bathroom or the kitchen while playing an online game. With the Shield, you are truly mobile (if only in your own house). Then you can continue gaming on Android games when you're away from your home network.
Casual Android gamers that want mobility will buy a tablet for the same price (or cheaper), or pick up an Android running system with hard buttons (like the Archos GamePad or the JDX S7300). Nobody is going to drop money on the Shield to play Angry Birds, or whatever the current game fad is.
With the cost of the OUYA it appeals to all types of Android gamers that just want to play with a controller, on a full size display. $100 is $100 though, so it's not like everyone can just buy the OUYA on a whim, but it is much easier to justify dropping $100, especially when you factor in other features besides gaming (streaming media, playing movies/TV from removable media, showing off all your terribly filtered food pictures to your family and friends on a big screen, and whatever else you decide to do with it).
We won't be seeing the latest and greatest SoC right after it drops, even on future refreshes of the OUYA. It's already been said that they won't build a successor until they can bring it to the market at a sub-$100 price. If they wait for the Tegra 5, we won't be getting a hardware refresh until the Tegra 5 has been on the market for at least 8-10 months (or longer).
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