I know that GigaByte makes good boards, I have never owned an Nvidia chipset board though. The SLI board (like the name says) will allow you to upgrade to two GPUs if you ever want to. The GigaByte only has one PCIe x16 slot however, so dual GPU is out of the question if you ever want to go down that road.
That might help you decide, it may not. I think either one would be a solid board so it comes down to the features that you want.