I think you're right Stephanie. I've always considered the -se conjugation to be used in writing rather than conversationally :)

I personally prefer the '-se' option because it is so different from the future tense: [i.e., there could be confusion if you failed to notice that the 'tilde' was missing when encountering an '-ra' version - or, in conversation, through not realising that the last '-a' syllable was in fact unstressed]. This mistake has actually occurred [unwittingly] a few times with other students in our class here in SE Spain. It could be said that the '-se' form stands out better, because of its distinctive 's' sound.

I found it interesting to read about the 'rivalry' between the two forms in Lola Pons's book "El Árbol de la Lengua" (pages 95-97). The imperfect subjunctive used to be constructed only with '-se', never with '-ra' > because at that time the '-ra' suffix denoted the pluperfect form of the verb. They both developed that way from Late Latin, when imperfect subjunctives ended in '-isset', and pluperfects in '-erat'.

The change-over [in Castilian] did not actually take place until the 15th and 16th centuries - when the '-ra' option established itself as an alternative way of expressing the imperfect subjunctive.

I think I'm right in saying that the above-mentioned switch did not take place in Galician or Catalán.

Gracias Inma, 

I'm glad that I'm clear on that one now :)

Having lived in the Canary Islands, your explanation here makes perfect sense to me. 

Saludos :)

Ah, yes David, 

I agree, less risk of mistakes if we use the -se ending :)

Thank you David for that very interesting info you've shared here. I really love to read and learn more about the history of language.