I have taken the words spoken by both Obama and Clinton during the Pennsylvanian Democratic debate held on April 16th, 2008 and constructed from them a Document Contrast Diagram. See the link for a description of how to interpret the diagram.
It shows that they spoke roughly the same number of words but with Obama speaking slightly more. Both were slightly positive in overall emotional tone with some areas of negativity related to guns and security for Clinton and taxes for Obama. There was a great deal of overlap in the words used by the two speakers with the words 'kind', 'Democrats' , 'important', 'country', 'make', 'work', 'president', 'can', 'take' , 'right', and 'guns' being frequently used by both. 'Know' was used a lot by both but more often by Clinton. They both spoke each others names much more than their own but Obama used Clinton's name more often than the reverse.
Key words used frequently and uniquely or much more often by Obama included 'true' , 'statement' , 'economic' , 'issues', 'election', 'confident', 'George' , 'American', 'policy', 'politics', 'income', 'change', 'General', 'ideas', 'Chicago', and 'individuals'. Words used frequently and uniquely or much more often by Clinton included 'decisions', 'stay', 'withdraw', 'Iran', 'failed', 'begin', 'world', 'military', 'best', 'York', 'administration', 'Philadelphia', 'impose' , 'order', 'police', and 'oil'.
Pennsylvanian Debate Contrast Diagram (click to see larger version)