Go with the Nvidia GeForce GTX 970 Zotac AMP! Extreme 4GB Edition. It is much better than Gigabyte, infact it is the best in the 970 series. It has the best cooling solution with triple silencer and 3 fans and 5 copper heatsinks.
Here is a comparison between Nvidia GeForce GTX 970 4GB Gigabyte WINDFORCE Video Card and Nvidia GeForce GTX 970 Zotac AMP! Extreme 4GB Edition :
In terms of overall gaming performance, the graphical capabilities of the Nvidia GeForce GTX 970 Zotac AMP! Extreme 4GB Edition are marginally better than the Nvidia GeForce GTX 970 Gigabyte G1 Gaming 4GB Edition.
The GTX 970 has a 25 MHz higher core clock speed and the same number of Texture Mapping Units as the GTX 970. This results in the GTX 970 providing 2.6 GTexel/s better texturing performance. This still holds weight but shader performance is generally more relevant, particularly since both of these GPUs support at least DirectX 10.
The GTX 970 has a 25 MHz higher core clock speed than the GTX 970 and the same number of Render Output Units. This results in the GTX 970 providing 1.6 GPixel/s better pixeling performance. However, both GPUs support DirectX 9 or above, and pixeling performance is only really relevant when comparing older cards.
Both the GeForce GTX 970 Gigabyte G1 Gaming 4GB Edition and the GeForce GTX 970 Zotac AMP! Extreme 4GB Edition were released at the same time, so are likely to be quite similar.
Both GPUs exhibit very powerful performance, so it probably isn't worth upgrading from one to the other, as both are capable of running even the most demanding games at the highest settings.
The GeForce GTX 970 Gigabyte G1 Gaming 4GB Edition and the GeForce GTX 970 Zotac AMP! Extreme 4GB Edition have the same amount of video memory, but are likely to provide slightly different experiences when displaying game textures at high resolutions.
The GTX 970 has 6 GB/sec greater memory bandwidth than the GTX 970, which means that the memory performance of the GTX 970 is marginally better than the GTX 970.
Both the GeForce GTX 970 Gigabyte G1 Gaming 4GB Edition and the GeForce GTX 970 Zotac AMP! Extreme 4GB Edition have 1664 Shader Processing Units. Having the same number of SPUs and using the same architecture means that the performance they offer can be compared by looking at the memory bandwidth, Texture and Pixel Rates. In this case, the GTX 970 has 2.6 GTexel/s better Texture Fill Rate, 1.6 GPixel/s better Pixel Fill Rate, and 6 GB/sec greater memory bandwidth, so should have a marginally faster performance than the GTX 970.
The GeForce GTX 970 Gigabyte G1 Gaming 4GB Edition requires 150 Watts to run and the GeForce GTX 970 Zotac AMP! Extreme 4GB Edition requires 175 Watts. I would recommend a PSU with at least 550 Watts for the GTX 970 and a PSU with at least 600 Watts for the GTX 970. The GTX 970 requires 25 Watts more than the GTX 970 to run. The difference is not significant enough for the GTX 970 to have a noticeably larger impact on your yearly electricity bills than the GTX 970.