quality is subjective and difficult to assess, but the larger the diaphragms, the louder and deeper the possible bass. between two round speakers it is trivially easy to say that the the 6.5 will out-rock the 5.25. i would likewise expect a 6x9 to do better than the 6.5. how much better? not that much, probably, but i would always err on the larger side where there is room to accomodate it.