Hello there masked. Although the skylake line is (theoreticcaly) far more powerful than it's counterpart, there is still a distinctive difference between the 4790k and 6600k. The 4790k and 6600k both have 4 cores, although the 4790k has hyper-threading, and is clocked a bit higher. It truly depends on what you need either one for, if your workflow is more geared towards multithreaded workloads you can't go wrong with the 4790k. Although if you are simply playing games, the 6600k is a more cost effective solution. Considering that your friend would like to play games, process videos, and livestream, I would personally go with the 4790k, it being more powerful.