Either one can work, but I think text might be more versatile as long as it is BOLD or different color so that it STAND OUT. You see, in my experience people tend to ignore text especially if is long; so having something that stand out it will make them pay attention.
The problem with clickable notification is that people might not know they can click on it especially if you are using non-clickable notification throughout the game. How can you know the difference? Of course you can say "click here" in your notification text but that might defeat the purpose of having a clickable notification vs clickable text. Also clicking on the X button to close the notification, might trigger clicking on the notification it self.
So over all I think text it's a better approach.
As for the go to link/click, I think invoking a function is the best use, since is the most versatile option out there. This way the notification can link to things inside the game as well as outside the game (outside URL).
Example: Click here for more details on how to discover a secret weapon, or whatever! OR - Click here to brag about this on facebook.