When it comes to real-time, time-sensitive notifications, they both do the same thing. It is easy to argue that the watch form factor is actually a lot more feasible, and could see greater adoption in the consumer market.
First, you have to wear Google Glass on your face. You might as well get a Mike Tyson tattoo while you are at it. Unless you are in the market for a face-mounted camera (which I guess some people might be), a watch makes more sense -- the only advantage Google Glass has is a camera.
The sheer number of important time-sensitive notifications that people get (not including email) is quite low, and doesn't warrant wearing something on your face all day long. Even using "Google Now" can be embarrassing for some people -- since the low frequency of time-sensitive information people need makes one feel boring.
So, if this "real-time, time-sensitive" device is only going to be actively used for around 3 minutes a day, something on your wrist that won't get the wearer accused of taking sneaky pictures when they actually aren't, and still really good at alerting you (by vibrating) seems like something most people would be quite comfortable with.
But do you need something like this at all? Well, it's true that these types of notifications are also available on your phone, but with a watch, you don't have to worry about missing anything -- A buzz on your wrist is far more effective, and less annoying, than a buzz/ding in your pocket.