I don't know what you mean by a horizontal or vertical circle, but i do know that angular velocity is expressed in or radians or degrees over time.
The only thing I can think of is if, for example, you are swinging a mass on the end of a string. In the horizontal circle the angular velocity will remain constant because the tension is constant. In a vertical circle, the tension is not constant (check out the tension at the top and bottom of the motion and you will see that the tensions are different.) So in the vertical circle the angular velocity is not constant.
-Dan