If you want to use 8 GB RAM for your laptop of PC, it is always better to go for 1 stick for 8 GB RAM. Because 2 RAM stick together may cause crashing of any running program some times.
Ideally 2 RAM stick with same frequently will not give trouble, but it happens some time that even if both RAM is not faulty when using single, if you put together in same PC you will get RAM trouble.
This trouble may happen because of some frequency mismatch, two RAM are connected with each other with circuit of motherboard. The motherboard is coming in between the two RAM, it may create some problem. It is possible that problem occurrence frequency may be very low. But even low trouble frequency can be avoided by using single stick of RAM only.
If you have option to select 1 stick of 8 GB RAM or 2 stick of 4 GB RAM, go for 1 stick of 8GB RAM.
You may think that even if one RAM of 4 GB will become faulty, you will have another 4 GB RAM available. But it doe not matter. If RAM is healthy, it will not become faulty easily over long period of time.