The 6970 isn't a new architecture. It's still the same architecture with more processing cores.
We won't know anything about the performance of either card though until we get official in-game benchmarks.
The hd6970 is actually quite modified from the old architecture and has less shaders. It's not southern islands (which was pushed back the hd7xxx due to 32nm being dropped), but it's a halfway house between evergreen and southern. It's expected to be competing at least well with the gtx580. Currently we do not know about this cards performance at all, the scarce few leaks seem to indicate around 20% faster than a gtx480. Personally i'd hope for more considering the size of the die compared to the hd5870 and the larger number of shaders of the hd6870, but ah well.
Also, the gtx580 is, at most, 15-20% faster than the gtx480, fact. The architecture is identical bar the stripping down of the double float precision aspects of the chip. So the only difference is the extra few CUDA cores and a 10% core boost. Therefore we can logically expect a 20% performance boost at best from the card. It's easily the most boring out of the two cards because we know what it'll do before it even releases. It'll also cost £399, ouch.
EDIT: It's around 10-15% faster, it performs exactly the same as the hd5970.
If both of these predictions turn out to be true (it's likely) then we'll see some fantastic competition though! Also, Nvidia have made some progress with their tessellation uses too (in software), there's a great tessellation demo around atm with an alien

Finally, the only reason why nvidia involvement would be bad is if the purposely try to nerf AMD performance, and at this point they have their backs pressed firmly against a wall in this market (80:20 AMD:nvidia ratio for dx11 cards), so i could see them trying it as they've done it before several times. That's why i'm a little skeptical. I'd be far more comfortable if AMD had paid the $2m because they've never blatantly nerfed nvidia cards in games they've supported.