A vertical disc head on with a horizontal is all just a big gamble really. If they are of similar specs its all about whos tooth hits the other disc first!
As Gary says, a vertical disc is good for flipping a robot over aswell, and it often causes damage to the underneath/bottom corners of the robot which useally twists the chassis alot easier than a horizontal spinner. But then again, a horizontal spinner is usually better against wedge robots head on - and it looks really spectacular when both robots spin across the arena after a good hit