Both. Leveling is odd in D2. Your character doesn’t level at all, he just gets better gear with increasingly higher “light scores.” It goes up fairly quickly doing any content at all, then it slows down quite a bit at the soft cap.
Most of the content is horizontal in terms of power growth. Each expansion has a host of legendary and exotic gear pieces and ways to farm them. Until you get to the very end of the endgame, most of that gear remains viable, and you have enormous latitude to choose based on playstyle. There’s a ton of fun stuff to chase.