Warning: I am no expert on the topic, and my answer might be inaccurate, or downright wrong.
Looks like they're both sensor fusions of h/w sensors- accelerometer, gyro, and magnetometer. (https://blogs.msdn.com/b/b8/archive/2012/01/24/supporting-sensors-in-windows-8.aspx)
According to http://msdn.microsoft.com/library/windows/apps/hh465294.aspx, "Developers typically use this data to control complex games."
Also, http://msdn.microsoft.com/en-US/library/windows/apps/hh465282.aspx says that
The mathematics behind quaternions is fairly exotic in that it involves the geometric properties of complex numbers and mathematical properties of imaginary numbers, but working with them is simple, and frameworks like DirectX support them."
It seems that OrientationSensor is a convenience API for those few cases where it indeed proves to be convenient.