The speed is the same, it doesn't matter the size. The number of modules usually affects price as larger capacity modules cost more. Running multiple modules in 'channels' can improve performance slightly, but not as much as more ram.
2x 4GB 1600Mhz is the same speed and capacity as 1x 8GB 1600Mhz, but may cost less.
2x 4GB 1600Mhz may work faster than 1x 8GB 1600Mhz because of dual channels (1x module per channel).
2x 2GB 1600Mhz may work faster than 1x 8GB 1600Mhz because of dual channels, but the system will be slower overall because it is 1/2 the capacity.