I'd say read either one. If you want to read the best known of all her stories (and, I suppose, the most popular), then read Pride and Prejudice. For me, though, it wouldn't matter which one. I suppose that is because whenever I tell someone to read an Austen novel, I always suggest Persuasion. In my opinion, it is far superior to her other stories (though I enjoyed them all). Of course, I always follow the Persuasion comment with "read P&P if you want to read the popular one". :)

