Both are gorgeous bags, but the Ali is considerably larger than the shoulder bag. It was simply too big for me. I think it just depends on how much you carry and how you like the bag to fall, as the strap on the Ali is longer, too. I think you just have to try them on to decide which is right for you and feels most comfortable for you.
I bought the Legacy Hippie as a compromise between the Ali and the shoulder bag, but now I am considering getting the shoulder bag, too!