Two different forms of Subjuntivo Imperfecto & Pluscuamperfecto?

Hey all! I always see two different forms of conjugation for Sub. Imperfecto and Sub. Pluscuamperfecto. For example, conjugating the verb “Tener” in first person (yo);

Subjuntivo Imperfecto: Tuviera / Tuviese

Subjuntivo Pluscuamperfecto: Hubiera tenido / Hubiese tenido

I’ve done a lot of research on this before I decided to post here. All sources say they’re both grammatically correct, they’re just alternatives.

But which one is really used? More common? Does it differ from culture to culture?

Thank you in advance!