The best performance-to-price ratio in that performance class is completely owned by the RX 6950 XT.
TechPowerUp's GPU Database entry for the RX 6950 XT shows the RTX 4070 Ti to be all of 4% faster on average which is almost a statistical tie. Hell, I've seen different card models with the same GPU that have a bigger difference than that.
There are advantages to either card an only you can say which matters most to you.
Advantages for the RTX 4070 Ti:
1) The 4070 Ti will have much better performance with RT turned on than the 6950 XT. It's not that the 6950 XT is unusable with RT, but there's no question that the 4070 Ti will be much faster. If you care about RT performance, then this is definitely the card for you.
2) The TDP of the 4070 Ti is 50W lower than that of the 6950 XT. The TDP of the 6950 XT is 335W while the TDP of the 4070 Ti is 285W. This one probably doesn't matter because if you cared about power draw, you wouldn't already be looking at a card with a TDP of 285W.
3) If you employ professional graphics applications, you might need CUDA or you might use software that requires nVidia hardware. If that's the case, then the RTX 4070 Ti is the card for you. It's not like ATi hardware can't do this, it's just not nearly as good in most cases. If all you do is game or stream, then this isn't an advantage.
Advantages for the RX 6950 XT:
1) The 6950 XT has 16GB of VRAM while the 4070 Ti has only 12. It's not like the RTX 4070 Ti will be obsolete anytime soon (it's not like it has only 8GB) but the 6950 XT will definitely live longer. If you want your card to last as long as possible, this is the card for you. If you don't care about that and upgrade often, then this advantage doesn't apply to you.
2) The 4070 Ti is 33% more expensive despite being only 4% faster than the 6950 XT:
ASRock Radeon RX 6950 XT Phantom Gaming 16GB - $600
ASUS GeForce RTX 4070 Ti TUF 12GB - $800
For someone who has to save up for a video card, this difference is HUGE because there's always something else that the extra $200 (+whatever sales tax you pay) would be better spent on than a video card.
Unless ALL of the 4070 Ti's advantages apply to you, there's no question that the RX 6950 XT is the better buy, especially for someone who has to save up for a video card (like you and me both).
Theoretically, your PSU should be more than enough for any high-end card but that depends greatly on the make and model. If it's a recognised name like Corsair, EVGA, Seasonic, BeQuiet!, Silverstone or Antec, you should be just fine.