IMO the Gorilla websocket package should be preferred to the one in /x/net specifically because it has a superior API (it better represents the message-oriented nature of the protocol). There was a thread a while ago about fixing the /x/net API to make it more in line with gorilla/websocket, but that didn't seem to go anywhere.
The Gorilla implementation also seemed more fully featured the last time I looked into it (which admittedly was a while ago).
I've built sites using /x/net/websocket but all my future websocket projects will use gorilla/websocket.
-Caleb