Difference between -iera and -iese ending of the imperfect subjunctive?

Question

There are two forms of the imperfect subjunctve in use, for example, pudiera, pudieras, pudiera, pudiéramos, pudierais, pudieranand pudiese, pudieses, pudiese, pudiésemos, pudieseis, pudiesenI believe that these have identical meaning (please correct if this assumption is wrong), but was there any difference historically in their origin and/or are one or other of the forms more or less commmon in different parts of the Spanish speaking world -- which may explain the historical origin.

Answer 1

There is no difference in meaning; however, -ara/iera forms are more commonly used in spoken language. On the other hand, we can find more -ase/iese forms in the books.