The GTX 970 is on par with the 3gb GTX 1060 while the 6gb GTX 1060 matches the GTX 980. AMD cards that are within the performance levels of the GTX 970 and 3gb GTX 1060 are the R9 390 and RX 480. The R9 390x is between the GTX 970 and GTX 980. The performance difference between the GTX 970/3gb GTX 1060 is 10%-12% slower than the GTX 980/6gb GTX 1060. The 3gb GTX 1060 has had 11% of it's shaders cut in comparison to the 6gb version, and performance figures generally follow this number.
I didn't bother to put the GTX 780ti in that list because of poor driver support. Typically the GTX 780ti is on par or slightly exceeds the GTX 970. This might be true with older games that have alredy been refined but this isn't the case with newer games.
The other thing you might find as a detour about the GTX 970 is it only has 3.5gb of useful memory.
I buy and sell used hardware on both ebay and craigslist. $245 for a GTX 1070 is a great deal but $175 for a GTX 970 isn't. Just the other day I saw a GTX 970 going for $120 and I honestly wouldn't pay a penny more for that card. Newer EVGA cards will be worth a little bit more than their Asus, Gigabyte, and MSI counterparts because EVGA cards have a 3-year Transferable warranty. However, since the GTX 970 was released in 2014, many of these EVGA GTX 970 cards have an expired warranty. A GTX 1070 on the other hand will be 2 years old at the most.
The GTX 1070 is roughly 55% faster according to techpowerup. Both cards have roughly the same power draw and they should be paired with at least a decent 550w power supply. I don't generally recommend a 500w unit because there isn't a 500w unit that's not a ticking timebomb.
When you crunch those numbers under a performance per dollar figure. You're actually getting more bang for your buck by spending more on the GTX 1070.
As far as performance goes. The GTX 970 won't max out new games at 1080p but you can at least play them at high settings. The GTX 970 can't always do Ultra. The 1070 on the other hand will max out pretty much everything at 1080p while giving you around 75-90fps depending on the game. The GTX 1070 was considered to be overkill for 1080p, but more suitable for 1080p 144hz setups and 1440p. Nowdays newer games are pushing those limits.
Time is on your side. There is a possibility that Graphics cards prices could drop an additional 10% to 20% this month alone. Suppliers are backstocked with cards and it's tough to move the supply when prices of cards still remains high. There are rumors that Nvidia has put off the release of the 11 series because of backstock issues with the 10 series. You don't need to be in a hurry or anything. One last detail. Avoid any card with a single fan or blower cooler. These cards always have poor thermal results at stock speeds.