STERN have said over n over again that the PRO models have been designed to give shorter ball times, more random ball play, and lower scoring for the coin op market. Accordingly, I reckon the PRO from what STERN are saying is generally a more challenging version of the games released.
AC/DC Pro is a example, very fast game, and Transformers Pro plays faster than the LE etc. I think the PRO versions are designed to be harder so u chew thru coins playing them onsite etc. Thats what I meant