It depnds on your hardware and there is more involved than just RAM size. If your memory controller is capable of running dual cahnnell and single channell at the same time option 2 would be faster than option one assuming that all latencey settings are the same. With option 2 (if your controller is capable) you may have 8GB ram in dual channel and 4GB in single channel. if your Memory controller is incapable of running the memory in this fasion then option 1 & 2 will be the same speed.
If all the same specs on the sticks and they will play together, option 2 will always be faster overall....If they play together, the worst scenario is 8GB vs 12GB in single channel - 12 is better overall.....if the rig supports Flex mode then you have 8 in dual and the odd 4 in single, so either way 12 will be better overall than 8GB