I can definitely tell you the game plays differently when playing 15 min qtrs. Bear in mind I'm going off maybe 2 updates ago (I believe the zero MM penalty iteration) but one thing I noticed is that big plays were more prevelent & came closer together on 9 mins.
Not an ideal sample size game wise as I didn't play full games but I saw the difference in a half of football each w/9 min vs 15 mins w/acc clock. Giants (me)/Doplhins for both games.