I'm a little biased towards Gigabyte as i've always had their products and never had a single problem with them.
But that's just my opinion on it.
If you want facts,
Zotac EXTREME ( is that your option? ):
Base: 1771 MHz
Boost:1911 MHz
Gigabyte:
Base: 1721 MHz
Boost: 1860 MHz
Although gigabyte is a bit slower, those 50Mhz won't make that difference in terms of FPS gain, so i wouldn't worry about it.
So, it mainly goes within your own preferences, get the one that you think is best looking/fit more properly in your pocket as both will have the same performance overall