6970 has better value than the 570, but the 570 is ever so slightly faster. Definitely not enough to justify it's increased price, not even enough to tell the difference between it and the 6970 most of the time, but a difference none the less.
Although the 6970 is still better because of it's RAM capacity advantage. It's even better for multi-GPU setups. The 560 TI is a little cheaper and uses less power than the 6950, it's not just slower than the 6950. The two are listed as having similar value by Tom's.
In general, Nvidia has less value than AMD so it's almost pointless to buy Nvidia right now. Nvidia doesn't have low end cards and their mid range cards suck so they only compete in the high end, where Nvidia tends to have less value than AMD.
If value is the main thing in question, go for an older model like the GTX 400 cards or the Radeon 5000 cards with similar performance to what you want. I guarantee that the Radeon 5970 will outperform any similarly priced cards. The last time I saw it on Newegg it was selling for $330 and it competes with the GTX 580.