Generally 2x the performance for just over 2x the cost. Sure this means that the 7970 is better 'value' but it is not 'capable' for todays games. So you might have something cheap but kind of pointless.
The 970 since it's one tier higher than the 7970. You're also forgetting that the equivalent GTX970 release was the R9-290 which is a generation after the 7970. You're also forgetting that the GTX970 is more power efficient thus lower power draw but that doesn't mean you can run it off a bad PSU.
Generally 2x the performance for just over 2x the cost. Sure this means that the 7970 is better 'value' but it is not 'capable' for todays games. So you might have something cheap but kind of pointless.