If you don't know, then just buy whats cheaper for you. Go for MSI if you want quiet operation and go for Gigabyte if you want overclock and cooler operation.
I think both are great. I prefer the MSI cards, but anyone who bought the Gigabyte windforce G1 say that it's the best GTX970.
Do as turbopixel said, buy the one you like more visually or is cheaper.