In theory:
Short passing should increase possession and reduce number of shots
Long passing should result in lower possession and shots off of counterattacks.ality.
Mixed would be normal ment

In actuality, I've never noticed any difference whatsoever using short passing. I've succesfully used long passing against teams stacked at midfield - I use a 3-6-1 V (e.g. 3n-1-2n-3w-1) - resulting in more shot opportunities. (Anecdotal evidence, of course - I haven't done any scientific studies)