To this point, I'm not sure if Stormblood has better story or flow than Heavensward, but I can surely say that it maintains that height that was set by the last game, so I think is just a matter or preferences.
The main story of this expansion is about revolutions and geopolitical stuff, if you're like me and like that kind of thing then this is going to be a big hit, but, that's not to say that it's a bad story, it's a great one but certainly is a different beast than Heavensward, this time you focus on other characters (new and old) and it still delivers in character development, the main antagonist while maybe a little one-note it does its job and does it amazing, I'm not sure if its the acting (voiced by the incredible Kosuke Toriumi) or the writing but I was so invested in him.
By this point you know how is this, you go to one map, uncover it by quests, beat some dungeons and bosses and repeat, is more or less the same, de difference is that the boss and dungeon mechanics were tightened, making to really interesting and challenging battles.
As far as music goes XIV always has had a very good selection, but I think this time the composers took it to the next level, there are some tunes that I will listen for the years to come, I'm sure.
While is debatable if its better than its predecessor, Stormblood maintains that incredibly high bar set by Heavensward, so I think you'll not disappointed, as, for me, I will always remember that final moment when you finally finish the last boss battle, it was a truly beautiful and powerful moment.