Let's compare the story of 3.0 and 4.0
I'll tell you straight up that I like Heavensward story better than Stormblood. Here are the reasons why :
1. SB had such a huge cast of side characters and maybe this is the reason why they feel under-developed. Listing some characters which are introduced in stormblood; The resistance commander (forgot his name), M'naago and the other liutenant, Gousetsu, Yugiri, Hien, Magnai, the black female aura (geez i'm so bad at remembering names), Zenos, Fordola, The evil lady, etc, and most importantly Lys. I would argue that only Lys that had a significant character development here.
Let's look at HW; Aymeric and his Girl "Liutenant" Friend, Ysayle, Estenien, Nidhogg, H'vaelgr? H'valegr?, Archbishop Thordan and his knights (basically they are one entity), etc. We see how Ysayle transition from refusing to work with the Ishgardian to a more open and diplomatic person. We see how Estenien is slowly corrupted by nidhogg's eye. The point is, maybe I prefer less new side characters and more deep development of that new side characters.
Now c'mon, who cried when the other liutenant or the resistance commander died? I didn't. But, I cried a little when Ysayle... ermm, paid the ultimate price.
2. SB character are so 1 dimensional. Character is only good or bad, there is no moral complexity in it, well excepts for fordola and the xaela. The rest is just so simple and almost boring (Lys included). Well maybe the Domans war fatique was also intriguing.
Compare that with HW. Nidhogg was antagonized for his revenge but we can sympathize with him because the hideous things that man had done to dragons and especially his sister. We get intrigued by Thordan's goal of obtaining the eye to be the judge of good and evil himself to bring order and peace. There is a moral ambiguity and complexity for every character. For me, It's a "hate to hate and hate to love" relationship with the HW characters.
3. SB build up to the end fight is basically, "Hey, here's a dungeon where you beat the main villain of the story. Oh wait, he has a pet dragon that he merges with and now you have to kill that too". Honestly, Shinryu was the last thing I expected when I was going to the final trial. Let me clarify that I loved the Shinryu sudden reveal moment. But, the way it is presented, with Shinryu completely detached from the rest of the story, it is just shocking and nothing else. I seriously tought that if fordola was the last boss of Ala Mhigo dungeon and then we fight Zenos with him becoming some primal (well, I guess shinryu would also be fine in this case), it would be much better.
HW build up on the other hand was so "expectation building". When you first play the game, you expect that nidhogg will be the opponent you will fight as the final boss. You see Thordan and you wonder, "Do I get to fight him?". Then you see Lahabrea and thought to yourself, "Oh geez maybe he is the final boss?". Then it gets clearer that Thordan and his... ehm... KNIGHTS was up for something nasty. Hmmmm..... Knights of the Rounds? That has to come up at some point. At the 50% story mark you killed nidhogg, you crossed nidhogg off the checklist. Then the last dungeon ARF, final boss Lahabra and his girlfriend. Wait a minute, no Knights of the Rounds? OH THERE IT IS. And the music is so divine especially the intro with just organ and man choirs. YASSSS
4. SB felt so rushed. I was getting invested with M'naago, Pipin and The resistance commander, nope we are going to Doma. Reason : to split the empire force thin. If you are going to do that, well, I don't know, create a bit of ruckus in Doma to lure some empire troops to Doma and attack the castle in Ala Mhigo and Doma at a relatively same time? But noooooooooo, the empire troop in Doma is already thin anyway and they were not planning to send reinforcement there. We came back to Ala Mhigo and we still have to take the Broken Castrum (do i still have to explain myself?) for them, the Castrum Abania for them, the pratoreat that got destroyed anyways and eventually the Ala Mhigo Castle itself. To the Alliance's credit, they did take Ala Giri on their own. (Sorry that it turned to be a bit of a rant). Anyway, SB story that also felt rushed :
- Xaelas (Azim Steppe)
- Ananta (The Peaks)
Maybe this is because we have two objectives in SB:To Free Doma and Ala Mhigo. In HW we only had one:To end the war in Ishgard. You really get to know how the war started, how each dragon siblings react to it and what caused them to react like that. You get to know the Ascian's goal (well, you basically should already know it in ARR), and how it seemed that Thordan was swayed but in actuality, he had an idea to how achieve peace. You get to know Aymeric's struggle as Thordan's son? And most importantly, you get to know estenien and ysayle through your journey in finding that peace. HW took the time to get you understand the character's backstories and personality.
---------------------------------------------------------------------
Now, this is only subjective point of view of the story of both expansion. I'm not saying SB is terrible bad. I'm just saying that HW is superior than SB in terms of story.
I have to admit though that dungeons and trials are a lot more fun in SB. I'm even surprised that they can top Ravana with all of the trials in SB. I mean, HW trial... ermm too forgiving and Bismark... ermm never mind.