I will mostly use the pc for gaming and sometimes video editing and streaming. Which should I get?Hard to say. If you need more than 8GB of RAM, then you need the 16GB kit.
If you use less than 8GB, the 2x4 (all else equal) would offer marginally better performance due to dual channel memory.
Ok I found one 2x8 is it good https://www.skroutz.gr/s/14226815/Mushkin-Blackline-16GB-DDR4-2400MHz-MBB4U240FFFF8GX2.html?utm_source=ios&utm_campaign=skroutz_shareWell is there any reason you cant get a 2x8GB kit?
For use like that, you would probably want the additional RAM. But I still dont really recommend a single stick.