Personally, I found the second game to be far superior to the first in the writing department. The first game felt more like the Prequels whereas the Sequel felt like the original trilogy. The Darker atmosphere really made the characters kind of pop out in a way. The Original seemed so happy almost all the way throughout and never gave you the sense of dread that the second game did when facing your mortal enemies the sith..
Since I've only play the first one I guess I'll have to pick the first one. I actually liked that it wasn't too heavy and dark where the story went. I prefer games not to be too serious.
I know a lot of people might disagree here, but for me it has to be the first. The writing, although less complex and in-depth, felt to be more proper Star Wars themed, and I got an overall sense of fun from it. The second one was also awesome to play through, but I just never felt the same sense of adventure, fun and wonder as with the first. Also for some reason the characters in the first are more memorable for me.
I like the second one a lot more. First one was great too but it wasn't really a fresh story. The second game I've never experienced anything quite like it, and it certainly made me think about things a lot more than the first one did.
I thought the first one was a way better game than the sequel. In fact, I thought the game was even better than the terrible Star Wars prequel movies. It had a great story and a really fun combat system.
The second one is much better in my opinion. Although at release it was bug riddled and even missed content, nowadays, most bugs have been cleaned out and the modding community has added to and even restored missing game content. If you have not played the second part yet, its the perfect time.