Apple will launch the iPhone 5S and it will contain a biometric sensor in the home screen button. The sensor will allow for the creation of novel applications that rely on fingerprint, heart rate, and body temperature. While the biometric sensor is interesting on the iPhone, it makes perfect sense in a device you wear 24/7 like the iWatch. It can track your body' signals in perpetuity and much more accurately than your iPhone.
By introducing the sensor on the iPhone first, Apple will have third party developers creating a suite of apps to capitalize on the new tech. These apps will be uniquely suited to the iWatch before it ever comes out.
Apple can launch the iWatch hardware later than competitors, even with a similar form factor, and be winning in the software front from day 0.
While there is a lot of hype around the hardware and form factor of the pending wearable releases, Apple will once again win due to the quality of the apps on the device.
Just like the launch of the original iPhone - to consumers it's all about apps.
Apple will have the best apps for wearables before they even launch one.