That's sort of like asking "which is faster, one cars or two?"
Horizontal scaling with two slow computers will perform poorly. Vertical scaling with some real IBM Iron is lightning fast.
There are too many variables to answer this question. Now, asking what the benefits of each approach might be is a different question.
Here's a tutorial about deployment to WebSphere, and the benefits and drawbacks associated with horizontal and vertical scaling. This might provide some pertinent information for you.
http://www.technicalfacilitation.com/get.php?link=deployment Cheers!
-Cameron McKenzie