What would be better for an online gaming pc ?
what motherboard?
why does the motherboard matter if it supports both ?
Depending on the number of channels it supports you will have your answer (what's why he asked you what mobo).
Most motherboards nowdays are Dual Channel, meaning you have the double broadband for the ram and the CPU to conect. If you install a 2x4gb memory kit correctly slots=1-2-3-4 Ram1_2_Ram2_4, you will have 1 DIMM (one stick) using all of the first channel, and the other DIMM using all of the second channel.
If you install 1x 8gb you are not using the second channel at all, meaning you are only using 50% of the avaible width.
If you have an older single channel mobo (those with only two slots) It won't matter at all.
If you have a Quad Channel or triple mobo, and want to use its full potential, you need something like 4xNgb or 3xNgb
Usually the 2 sticks is better, because you have a lower cas on the small sticks. That is not always the case though. Also the motherboard may prefer 2 sticks over just 1.
Dual Channel. You lose at least 15% of FPS if you only go for single channel. Or something like that. Just go for dual channel because of memory bandwidth.