Well there is no point in kindly stating why since both perform the same. Just get the one that appeals to you more. MSI, Asus STRIX and Gigabyte all look pretty sexy. Just be sure if u get the GIgabyte one that there is enough room in your case for it. As it's a bit bigger than the others. Also if you are still unsure and dont trust us ( ((((((( ), there are linustechtips and techoftommorow reviews on all cards.
Both manufacturers make good GTX 970, as well as Gigabyte.
The overall opinion is that MSI is the cheapest and has the best power delivery. They are pretty much the same performance-wise. The MSI one does not have a backplate. It also overclocks pretty good.
I would just say get the one that is cheaper and appeals to you with its looks. Whichever you choose, the differences would be very small to have a significant impact on performance.