This is a tough comparison to make considering that an iWatch is only a rumor at this point. Google Glass has already launched and people have started using the product. Nevertheless, I believe Apple will launch an iWatch or something along those lines because it simply makes far more sense in terms of company strategy.
First, why am I skeptical of Google Glass? Well it's an extension of your smartphone. It also works in conjunction with your smartphone and requires you to continually touch your face to do certain tasks. Further, when wearing the device, it's always on....why would you otherwise have it on your face. This will inevitably lead to information overload. Finally, after paying somewhere between 200 and 700 Euro for their smartphone (without carrier subsidy), will people be willing to spend another 1500 Euro on top? Sure, that price would likely come down but it's an additional purchase to the smartphone everyone will buy.
Hence, why believe more in an iWatch? I simply feel that Apple will want to disrupt again. People have also stopped wearing watches and that real estate is empty! Just as they changed the whole concept of smartphones with the iPhone (and tablets with their iPad), they too will want to change the idea of a watch. If I were to bet on this, I'd say they would put personal metrics capabilities into their watch. This has already been pondered elsewhere and it really makes sense. Allow and iWatch to work together with my phone and measure everything about me such as heartrate, calories burned, steps taken and so forth....as is the case now with the Jawbone Up, Fitbit or Fuelband from Nike. I'm sure there are additional features which Apple could throw into the package to make this a must have accessory.
Finally, I believe it would be something selling for around 500 Euro. Sure, that's a hefty price but think about it. Would you rather have additional information which is relevant to your health and well-being and have this work in conjunction with your smartphone or on its own? Or do you prefer to have a pair of glasses on bombarding you with even more SMS's, calls and information which you only sometimes need and have available anyway via your smartphone?
If I were Apple, I'd go with the iWatch and personal metrics vs information overload to the face!