The brand and cooling systems are NOT the main differences here, so let's break it down. I will now attempt a Venn diagram in bullet point format:
Specs shared by both the MSI and Gigbyte models:
- Nvidia GeForce GTX 1070 GPU
- 8GB 8008MHz GDDR5 Graphics RAM
- Display connectors (3x DP, 1x HDMI 1x DL DVI-D)
Specs specific to MSI model:
- Base clock: 1556 MHz
- Boost clock: 1746 MHz
Specs specific to Gigabyte model:
- Base clock: 1594 MHz (gaming mode), 1620 MHz (OC mode)
- Boost clock: 1784 MHz (gaming mode), 1822 MHz (OC mode)
What you are paying the extra $100 for is not just the cooling system, it's also the pre-overclocked card. You are getting a card with a theoretical maximum speed that is 76 MHz faster.
Recommendation (TL;DR): If you have the extra money and want the little extra bit of power, go for it. But being that you chose the 1070 instead of the 1080, I'm thinking that little bit of power isn't very important to you, and probably not worth the $100 investment.