The G.Skill DDR3 memory F3-10666CL7D-8GBXH is rated at 1333 MHz @ CAS Latency 7 @ 1.5 V.
The Samsung DDR3 memory MV-3V4G3(D) is rated at 1600 MHz @ CAS Latency 11 @ 1.35 V; and at 1333 MHz @ CAS Latency 9 @ 1.35 V.
For operation at 1333 MHz, the G.Skill memory has a lower effective latency, so is faster than the Samsung memory.
But the Samsung memory can run at a lower voltage of 1.35 V, compared to the higher voltage (1.5 V) of the G.Skill memory.
The Gigabyte GA-990XA-UD3 Rev3.1 motherboard, I think, only supports memory voltages from 1.5 V and up. So the Samsung memory can't be run at 1.35 V, but the Samsung memory can be run at 1.50 V instead.