Simple: buy both, but start with KOTOR I, lol.

Only played the first one, but the story line was so good and the open world aspect was so addicting that I barely noticed the bad gameplay

By the time you finish your deliberations, you will have spent enough man hours to have bought both at least twice over. Just go ahead and spend the money and enjoy.

Buy both and start with the first so you can understand the story. I’d say the story of the second one is better though.