Duffless is one of the first Simpson episodes that I ever watched so it holds a massive nostalgia factor for me, and it has a sweet and heartwarming ending which I'm a sucker for.
Bart vs Thanksgiving, however, I've always found to be a bit of a perplexing episode. It paints Bart in the wrong and selfish for not respecting Lisa's art piece, but Bart was carrying in the turkey, the important part of the Thanksgiving meal so it had to be on the table. I always got the feeling that Marge & Homer are at fault here for not trying to make a place for Lisa's art and the turkey, before the kids started fighting. I don't know. Something feels off about the episode. Maybe I just don't get it because I'm not American.
I prefer Duffless because of the A plot and the sweet ending, plus it shows about alcoholism. Bart's vs Thanksgiving is good, but has also his drawbacks, and i'm not fan of it, especially the Bart's nightmare.