970's main selling point is its lower power consumption and emitted heat, as well as newer tech with a bit more new features than the almost-a-year-old 290x, although the Red team is churning out new features for its GPU's as we speak, and the only remaining 970 exclusive, MFAA, isn't supporting many games at the moment.
That's on pure raw power grounds, individual games performance might differ, some favor AMD, some favor nVidia, not withstanding bad coding (for example, AC:U runs ok on nVidia cards, but it's totally botched on AMD cards. These are few and far in between, for now).
If you don't mind the extra power consumption and heat, and have a PSU good enough in both quality and wattage, go for whichever is cheaper.
what power supply need for both ?