-ra vs. -se endings in Spanish subjunctive?

To any native speakers or experts in the Spanish language- I was wondering, what's the difference between the -ra and -se endings in the imperfect subjunctive tense? I've seen and heard both of them used all the time, but I was wondering- is there a stylistic difference between them? Or any kind of preference depending on the country? I already know how to use both forms correctly- I'm pretty much fluent in the language- but I've always wondered if there's a difference between the two. Thanks in advance!