I already have a Yongnuo flash, just one of the fully manual ones. I also have a Nissin Di622, which has served me well for years.
Basically I wanted a TTL flash like my Nissin, and one that can be fired on remotes like my Yongnuo.
The Canon's also are able to be used in low light to aid focus, an area the Nissin fails at.
Then there's build quality, these flashes will be used for years so I need them to be built to a quality that will last, eventually they will become purely slaves but in the mean time they will be used heavily for on and off camera depending on the type of shoots I'm on.